Linux Virtualization - Sep 2018 - [patch 09/11] x86/vdso: Simplify the invalid vclock case

Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
implementation, which extended the clockid switch case and added yet
another slightly different copy of the same code.

Especially the extended switch case is problematic as the compiler tends to
generate a jump table which then requires to use retpolines. If jump tables
are disabled it adds yet another conditional to the existing maze.

This series takes a different approach by consolidating the almost
identical functions into one implementation for high resolution clocks and
one for the coarse grained clock ids by storing the base data for each
clock id in an array which is indexed by the clock id.

This completely eliminates the switch case and allows further
simplifications of the code base, which at the end all together gain a few
cycles performance or at least stay on par with todays code. The resulting
performance depends heavily on the micro architecture and the compiler.

Thanks,

	tglx

8<-------------------
 arch/x86/Kconfig                        |    1 
 arch/x86/entry/vdso/vclock_gettime.c    |  199 ++++++++------------------------
 arch/x86/entry/vsyscall/vsyscall_gtod.c |   55 ++++----
 arch/x86/include/asm/vgtod.h            |   46 ++++---
 arch/x86/kernel/time.c                  |   22 +++
 include/linux/clocksource.h             |    5 
 kernel/time/Kconfig                     |    4 
 kernel/time/clocksource.c               |    2 
 8 files changed, 144 insertions(+), 190 deletions(-)

Architectures have extra archdata in the clocksource, e.g. for VDSO
support. There are no sanity checks or general initializations for this
available. Add support for that.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 include/linux/clocksource.h |    5 +++++
 kernel/time/Kconfig         |    4 ++++
 kernel/time/clocksource.c   |    2 ++
 3 files changed, 11 insertions(+)

--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -241,6 +241,11 @@ static inline void __clocksource_update_
 	__clocksource_update_freq_scale(cs, 1000, khz);
 }
 
+#ifdef CONFIG_ARCH_CLOCKSOURCE_INIT
+extern void clocksource_arch_init(struct clocksource *cs);
+#else
+static inline void clocksource_arch_init(struct clocksource *cs) { }
+#endif
 
 extern int timekeeping_notify(struct clocksource *clock);
 
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -12,6 +12,10 @@ config CLOCKSOURCE_WATCHDOG
 config ARCH_CLOCKSOURCE_DATA
 	bool
 
+# Architecture has extra clocksource init called from registration
+config ARCH_CLOCKSOURCE_INIT
+	bool
+
 # Clocksources require validation of the clocksource against the last
 # cycle update - x86/TSC misfeature
 config CLOCKSOURCE_VALIDATE_LAST_CYCLE
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -937,6 +937,8 @@ int __clocksource_register_scale(struct
 {
 	unsigned long flags;
 
+	clocksource_arch_init(cs);
+
 	/* Initialize mult/shift and max_idle_ns */
 	__clocksource_update_freq_scale(cs, scale, freq);

Runtime validate the VCLOCK_MODE in clocksource::archdata and disable
VCLOCK if invalid, which disables the VDSO but keeps the system running.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/Kconfig       |    1 +
 arch/x86/kernel/time.c |   16 ++++++++++++++++
 2 files changed, 17 insertions(+)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -48,6 +48,7 @@ config X86
 	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
 	select ANON_INODES
 	select ARCH_CLOCKSOURCE_DATA
+	select ARCH_CLOCKSOURCE_INIT
 	select ARCH_DISCARD_MEMBLOCK
 	select ARCH_HAS_ACPI_TABLE_UPGRADE	if ACPI
 	select ARCH_HAS_DEBUG_VIRTUAL
--- a/arch/x86/kernel/time.c
+++ b/arch/x86/kernel/time.c
@@ -10,6 +10,7 @@
  *
  */
 
+#include <linux/clocksource.h>
 #include <linux/clockchips.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
@@ -105,3 +106,18 @@ void __init time_init(void)
 {
 	late_time_init = x86_late_time_init;
 }
+
+/*
+ * Sanity check the vdso related archdata content.
+ */
+void clocksource_arch_init(struct clocksource *cs)
+{
+	if (cs->archdata.vclock_mode == VCLOCK_NONE)
+		return;
+
+	if (cs->archdata.vclock_mode >= VCLOCK_MAX) {
+		pr_warn("clocksource %s registered with invalid vclock_mode %d.
Disabling vclock.\n",
+			cs->name, cs->archdata.vclock_mode);
+		cs->archdata.vclock_mode = VCLOCK_NONE;
+	}
+}

All VDSO clock sources are TSC based and use CLOCKSOURCE_MASK(64). There is
no point in masking with all FF. Get rid of it and enforce the mask in the
sanity checker.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |    2 +-
 arch/x86/kernel/time.c               |    6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -199,7 +199,7 @@ notrace static inline u64 vgetsns(int *m
 #endif
 	else
 		return 0;
-	v = (cycles - gtod->cycle_last) & gtod->mask;
+	v = cycles - gtod->cycle_last;
 	return v * gtod->mult;
 }
 
--- a/arch/x86/kernel/time.c
+++ b/arch/x86/kernel/time.c
@@ -120,4 +120,10 @@ void clocksource_arch_init(struct clocks
 			cs->name, cs->archdata.vclock_mode);
 		cs->archdata.vclock_mode = VCLOCK_NONE;
 	}
+
+	if (cs->mask != CLOCKSOURCE_MASK(64)) {
+		pr_warn("clocksource %s registered with invalid mask %016llx. Disabling
vclock.\n",
+			cs->name, cs->mask);
+		cs->archdata.vclock_mode = VCLOCK_NONE;
+	}
 }

The sequence count in vgtod_data is unsigned int, but the call sites use
unsigned long, which is a pointless exercise. Fix the call sites and
replace 'unsigned' with unsinged 'int' while at it.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |    8 ++++----
 arch/x86/include/asm/vgtod.h         |   10 +++++-----
 2 files changed, 9 insertions(+), 9 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -206,7 +206,7 @@ notrace static inline u64 vgetsns(int *m
 /* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
 notrace static int __always_inline do_realtime(struct timespec *ts)
 {
-	unsigned long seq;
+	unsigned int seq;
 	u64 ns;
 	int mode;
 
@@ -227,7 +227,7 @@ notrace static int __always_inline do_re
 
 notrace static int __always_inline do_monotonic(struct timespec *ts)
 {
-	unsigned long seq;
+	unsigned int seq;
 	u64 ns;
 	int mode;
 
@@ -248,7 +248,7 @@ notrace static int __always_inline do_mo
 
 notrace static void do_realtime_coarse(struct timespec *ts)
 {
-	unsigned long seq;
+	unsigned int seq;
 	do {
 		seq = gtod_read_begin(gtod);
 		ts->tv_sec = gtod->wall_time_coarse_sec;
@@ -258,7 +258,7 @@ notrace static void do_realtime_coarse(s
 
 notrace static void do_monotonic_coarse(struct timespec *ts)
 {
-	unsigned long seq;
+	unsigned int seq;
 	do {
 		seq = gtod_read_begin(gtod);
 		ts->tv_sec = gtod->monotonic_time_coarse_sec;
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -15,9 +15,9 @@ typedef unsigned long gtod_long_t;
  * so be carefull by modifying this structure.
  */
 struct vsyscall_gtod_data {
-	unsigned seq;
+	unsigned int seq;
 
-	int vclock_mode;
+	int	vclock_mode;
 	u64	cycle_last;
 	u64	mask;
 	u32	mult;
@@ -44,9 +44,9 @@ static inline bool vclock_was_used(int v
 	return READ_ONCE(vclocks_used) & (1 << vclock);
 }
 
-static inline unsigned gtod_read_begin(const struct vsyscall_gtod_data *s)
+static inline unsigned int gtod_read_begin(const struct vsyscall_gtod_data *s)
 {
-	unsigned ret;
+	unsigned int ret;
 
 repeat:
 	ret = READ_ONCE(s->seq);
@@ -59,7 +59,7 @@ static inline unsigned gtod_read_begin(c
 }
 
 static inline int gtod_read_retry(const struct vsyscall_gtod_data *s,
-					unsigned start)
+				  unsigned int start)
 {
 	smp_rmb();
 	return unlikely(s->seq != start);

It's desired to support more clocks in the VDSO, e.g. CLOCK_TAI. This
results either in indirect calls due to the larger switch case, which then
requires retpolines or when the compiler is forced to avoid jump tables it
results in even more conditionals.

To avoid both variants which are bad for performance the high resolution
functions and the coarse grained functions will be collapsed into one for
each. That requires to store the clock specific base time in an array.

Introcude struct vgtod_ts for storage and convert the data store, the
update function and the individual clock functions over to use it.

The new storage does not longer use gtod_long_t for seconds depending on 32
or 64 bit compile because this needs to be the full 64bit value even for
32bit when a Y2038 function is added. No point in keeping the distinction
alive in the internal representation.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c    |   24 +++++++++------
 arch/x86/entry/vsyscall/vsyscall_gtod.c |   51 ++++++++++++++++----------------
 arch/x86/include/asm/vgtod.h            |   36 ++++++++++++----------
 3 files changed, 61 insertions(+), 50 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -206,6 +206,7 @@ notrace static inline u64 vgetsns(int *m
 /* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
 notrace static int __always_inline do_realtime(struct timespec *ts)
 {
+	struct vgtod_ts *base = &gtod->basetime[CLOCK_REALTIME];
 	unsigned int seq;
 	u64 ns;
 	int mode;
@@ -213,8 +214,8 @@ notrace static int __always_inline do_re
 	do {
 		seq = gtod_read_begin(gtod);
 		mode = gtod->vclock_mode;
-		ts->tv_sec = gtod->wall_time_sec;
-		ns = gtod->wall_time_snsec;
+		ts->tv_sec = base->sec;
+		ns = base->nsec;
 		ns += vgetsns(&mode);
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
@@ -227,6 +228,7 @@ notrace static int __always_inline do_re
 
 notrace static int __always_inline do_monotonic(struct timespec *ts)
 {
+	struct vgtod_ts *base = &gtod->basetime[CLOCK_MONOTONIC];
 	unsigned int seq;
 	u64 ns;
 	int mode;
@@ -234,8 +236,8 @@ notrace static int __always_inline do_mo
 	do {
 		seq = gtod_read_begin(gtod);
 		mode = gtod->vclock_mode;
-		ts->tv_sec = gtod->monotonic_time_sec;
-		ns = gtod->monotonic_time_snsec;
+		ts->tv_sec = base->sec;
+		ns = base->nsec;
 		ns += vgetsns(&mode);
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
@@ -248,21 +250,25 @@ notrace static int __always_inline do_mo
 
 notrace static void do_realtime_coarse(struct timespec *ts)
 {
+	struct vgtod_ts *base = &gtod->basetime[CLOCK_REALTIME_COARSE];
 	unsigned int seq;
+
 	do {
 		seq = gtod_read_begin(gtod);
-		ts->tv_sec = gtod->wall_time_coarse_sec;
-		ts->tv_nsec = gtod->wall_time_coarse_nsec;
+		ts->tv_sec = base->sec;
+		ts->tv_nsec = base->nsec;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }
 
 notrace static void do_monotonic_coarse(struct timespec *ts)
 {
+	struct vgtod_ts *base = &gtod->basetime[CLOCK_MONOTONIC_COARSE];
 	unsigned int seq;
+
 	do {
 		seq = gtod_read_begin(gtod);
-		ts->tv_sec = gtod->monotonic_time_coarse_sec;
-		ts->tv_nsec = gtod->monotonic_time_coarse_nsec;
+		ts->tv_sec = base->sec;
+		ts->tv_nsec = base->nsec;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }
 
@@ -318,7 +324,7 @@ int gettimeofday(struct timeval *, struc
 notrace time_t __vdso_time(time_t *t)
 {
 	/* This is atomic on x86 so we don't need any locks. */
-	time_t result = READ_ONCE(gtod->wall_time_sec);
+	time_t result = READ_ONCE(gtod->basetime[CLOCK_REALTIME].sec);
 
 	if (t)
 		*t = result;
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -31,6 +31,8 @@ void update_vsyscall(struct timekeeper *
 {
 	int vclock_mode = tk->tkr_mono.clock->archdata.vclock_mode;
 	struct vsyscall_gtod_data *vdata = &vsyscall_gtod_data;
+	struct vgtod_ts *base;
+	u64 nsec;
 
 	/* Mark the new vclock used. */
 	BUILD_BUG_ON(VCLOCK_MAX >= 32);
@@ -45,34 +47,33 @@ void update_vsyscall(struct timekeeper *
 	vdata->mult		= tk->tkr_mono.mult;
 	vdata->shift		= tk->tkr_mono.shift;
 
-	vdata->wall_time_sec		= tk->xtime_sec;
-	vdata->wall_time_snsec		= tk->tkr_mono.xtime_nsec;
-
-	vdata->monotonic_time_sec	= tk->xtime_sec
-					+ tk->wall_to_monotonic.tv_sec;
-	vdata->monotonic_time_snsec	= tk->tkr_mono.xtime_nsec
-					+ ((u64)tk->wall_to_monotonic.tv_nsec
-						<< tk->tkr_mono.shift);
-	while (vdata->monotonic_time_snsec >-					(((u64)NSEC_PER_SEC) <<
tk->tkr_mono.shift)) {
-		vdata->monotonic_time_snsec --					((u64)NSEC_PER_SEC) <<
tk->tkr_mono.shift;
-		vdata->monotonic_time_sec++;
+	base = &vdata->basetime[CLOCK_REALTIME];
+	base->sec = tk->xtime_sec;
+	base->nsec = tk->tkr_mono.xtime_nsec;
+
+	base = &vdata->basetime[CLOCK_MONOTONIC];
+	base->sec = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
+	nsec = tk->tkr_mono.xtime_nsec;
+	nsec +=	((u64)tk->wall_to_monotonic.tv_nsec <<
tk->tkr_mono.shift);
+	while (nsec >= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
+		nsec -= ((u64)NSEC_PER_SEC) << tk->tkr_mono.shift;
+		base->sec++;
 	}
+	base->nsec = nsec;
 
-	vdata->wall_time_coarse_sec	= tk->xtime_sec;
-	vdata->wall_time_coarse_nsec	= (long)(tk->tkr_mono.xtime_nsec >>
-						 tk->tkr_mono.shift);
-
-	vdata->monotonic_time_coarse_sec -		vdata->wall_time_coarse_sec +
tk->wall_to_monotonic.tv_sec;
-	vdata->monotonic_time_coarse_nsec -		vdata->wall_time_coarse_nsec +
tk->wall_to_monotonic.tv_nsec;
-
-	while (vdata->monotonic_time_coarse_nsec >= NSEC_PER_SEC) {
-		vdata->monotonic_time_coarse_nsec -= NSEC_PER_SEC;
-		vdata->monotonic_time_coarse_sec++;
+	base = &vdata->basetime[CLOCK_REALTIME_COARSE];
+	base->sec = tk->xtime_sec;
+	base->nsec = tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
+
+	base = &vdata->basetime[CLOCK_MONOTONIC_COARSE];
+	base->sec = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
+	nsec = tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
+	nsec += tk->wall_to_monotonic.tv_nsec;
+	while (nsec >= NSEC_PER_SEC) {
+		nsec -= NSEC_PER_SEC;
+		base->sec++;
 	}
+	base->nsec = nsec;
 
 	gtod_write_end(vdata);
 }
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -5,33 +5,37 @@
 #include <linux/compiler.h>
 #include <linux/clocksource.h>
 
+#include <uapi/linux/time.h>
+
 #ifdef BUILD_VDSO32_64
 typedef u64 gtod_long_t;
 #else
 typedef unsigned long gtod_long_t;
 #endif
+
+struct vgtod_ts {
+	u64		sec;
+	u64		nsec;
+};
+
+#define VGTOD_BASES	(CLOCK_MONOTONIC_COARSE + 1)
+#define VGTOD_HRES	(BIT(CLOCK_REALTIME) | BIT(CLOCK_MONOTONIC))
+#define VGTOD_COARSE	(BIT(CLOCK_REALTIME_COARSE) | BIT(CLOCK_MONOTONIC_COARSE))
+
 /*
  * vsyscall_gtod_data will be accessed by 32 and 64 bit code at the same time
  * so be carefull by modifying this structure.
  */
 struct vsyscall_gtod_data {
-	unsigned int seq;
+	unsigned int	seq;
+
+	int		vclock_mode;
+	u64		cycle_last;
+	u64		mask;
+	u32		mult;
+	u32		shift;
 
-	int	vclock_mode;
-	u64	cycle_last;
-	u64	mask;
-	u32	mult;
-	u32	shift;
-
-	/* open coded 'struct timespec' */
-	u64		wall_time_snsec;
-	gtod_long_t	wall_time_sec;
-	gtod_long_t	monotonic_time_sec;
-	u64		monotonic_time_snsec;
-	gtod_long_t	wall_time_coarse_sec;
-	gtod_long_t	wall_time_coarse_nsec;
-	gtod_long_t	monotonic_time_coarse_sec;
-	gtod_long_t	monotonic_time_coarse_nsec;
+	struct vgtod_ts	basetime[VGTOD_BASES];
 
 	int		tz_minuteswest;
 	int		tz_dsttime;

do_realtime() and do_monotonic() are now the same except for the storage
array index. Hand the index in as an argument and collapse the functions.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |   35 +++++++----------------------------
 1 file changed, 7 insertions(+), 28 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -203,35 +203,12 @@ notrace static inline u64 vgetsns(int *m
 	return v * gtod->mult;
 }
 
-/* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
-notrace static int __always_inline do_realtime(struct timespec *ts)
+notrace static int do_hres(clockid_t clk, struct timespec *ts)
 {
-	struct vgtod_ts *base = &gtod->basetime[CLOCK_REALTIME];
+	struct vgtod_ts *base = &gtod->basetime[clk];
 	unsigned int seq;
-	u64 ns;
 	int mode;
-
-	do {
-		seq = gtod_read_begin(gtod);
-		mode = gtod->vclock_mode;
-		ts->tv_sec = base->sec;
-		ns = base->nsec;
-		ns += vgetsns(&mode);
-		ns >>= gtod->shift;
-	} while (unlikely(gtod_read_retry(gtod, seq)));
-
-	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
-	ts->tv_nsec = ns;
-
-	return mode;
-}
-
-notrace static int __always_inline do_monotonic(struct timespec *ts)
-{
-	struct vgtod_ts *base = &gtod->basetime[CLOCK_MONOTONIC];
-	unsigned int seq;
 	u64 ns;
-	int mode;
 
 	do {
 		seq = gtod_read_begin(gtod);
@@ -276,11 +253,11 @@ notrace int __vdso_clock_gettime(clockid
 {
 	switch (clock) {
 	case CLOCK_REALTIME:
-		if (do_realtime(ts) == VCLOCK_NONE)
+		if (do_hres(CLOCK_REALTIME, ts) == VCLOCK_NONE)
 			goto fallback;
 		break;
 	case CLOCK_MONOTONIC:
-		if (do_monotonic(ts) == VCLOCK_NONE)
+		if (do_hres(CLOCK_MONOTONIC, ts) == VCLOCK_NONE)
 			goto fallback;
 		break;
 	case CLOCK_REALTIME_COARSE:
@@ -303,7 +280,9 @@ int clock_gettime(clockid_t, struct time
 notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
 	if (likely(tv != NULL)) {
-		if (unlikely(do_realtime((struct timespec *)tv) == VCLOCK_NONE))
+		struct timespec *ts = (struct timespec *) tv;
+
+		if (unlikely(do_hres(CLOCK_REALTIME, ts) == VCLOCK_NONE))
 			return vdso_fallback_gtod(tv, tz);
 		tv->tv_usec /= 1000;
 	}

do_realtime_coarse() and do_monotonic_coarse() are now the same except for
the storage array index. Hand the index in as an argument and collapse the
functions.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |   20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -225,21 +225,9 @@ notrace static int do_hres(clockid_t clk
 	return mode;
 }
 
-notrace static void do_realtime_coarse(struct timespec *ts)
+notrace static void do_coarse(clockid_t clk, struct timespec *ts)
 {
-	struct vgtod_ts *base = &gtod->basetime[CLOCK_REALTIME_COARSE];
-	unsigned int seq;
-
-	do {
-		seq = gtod_read_begin(gtod);
-		ts->tv_sec = base->sec;
-		ts->tv_nsec = base->nsec;
-	} while (unlikely(gtod_read_retry(gtod, seq)));
-}
-
-notrace static void do_monotonic_coarse(struct timespec *ts)
-{
-	struct vgtod_ts *base = &gtod->basetime[CLOCK_MONOTONIC_COARSE];
+	struct vgtod_ts *base = &gtod->basetime[clk];
 	unsigned int seq;
 
 	do {
@@ -261,10 +249,10 @@ notrace int __vdso_clock_gettime(clockid
 			goto fallback;
 		break;
 	case CLOCK_REALTIME_COARSE:
-		do_realtime_coarse(ts);
+		do_coarse(CLOCK_REALTIME_COARSE, ts);
 		break;
 	case CLOCK_MONOTONIC_COARSE:
-		do_monotonic_coarse(ts);
+		do_coarse(CLOCK_MONOTONIC_COARSE, ts);
 		break;
 	default:
 		goto fallback;

Now that the time getter functions use the clockid as index into the
storage array for the base time access, the switch case can be replaced.

- Check for clockid >= MAX_CLOCKS and for negative clockid (CPU/FD) first
  and call the fallback function right away.

- After establishing that clockid is < MAX_CLOCKS, convert the clockid to a
  bitmask

- Check for the supported high resolution and coarse functions by anding
  the bitmask of supported clocks and check whether a bit is set.

This completely avoids jump tables, reduces the number of conditionals and
makes the VDSO extensible for other clock ids.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |   38 ++++++++++++++++-------------------
 1 file changed, 18 insertions(+), 20 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -239,29 +239,27 @@ notrace static void do_coarse(clockid_t
 
 notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 {
-	switch (clock) {
-	case CLOCK_REALTIME:
-		if (do_hres(CLOCK_REALTIME, ts) == VCLOCK_NONE)
-			goto fallback;
-		break;
-	case CLOCK_MONOTONIC:
-		if (do_hres(CLOCK_MONOTONIC, ts) == VCLOCK_NONE)
-			goto fallback;
-		break;
-	case CLOCK_REALTIME_COARSE:
-		do_coarse(CLOCK_REALTIME_COARSE, ts);
-		break;
-	case CLOCK_MONOTONIC_COARSE:
-		do_coarse(CLOCK_MONOTONIC_COARSE, ts);
-		break;
-	default:
-		goto fallback;
-	}
+	unsigned int msk;
+
+	/* Sort out negative (CPU/FD) and invalid clocks */
+	if (unlikely((unsigned int) clock >= MAX_CLOCKS))
+		return vdso_fallback_gettime(clock, ts);
 
-	return 0;
-fallback:
+	/*
+	 * Convert the clockid to a bitmask and use it to check which
+	 * clocks are handled in the VDSO directly.
+	 */
+	msk = 1U << clock;
+	if (likely(msk & VGTOD_HRES)) {
+		if (do_hres(clock, ts) != VCLOCK_NONE)
+			return 0;
+	} else if (msk & VGTOD_COARSE) {
+		do_coarse(clock, ts);
+		return 0;
+	}
 	return vdso_fallback_gettime(clock, ts);
 }
+
 int clock_gettime(clockid_t, struct timespec *)
 	__attribute__((weak, alias("__vdso_clock_gettime")));

The code flow for the vclocks is convoluted as it requires the vclocks
which can be invalidated separately from the vsyscall_gtod_data sequence to
store the fact in a separate variable. That's inefficient.

Restructure the code so the vclock readout returns cycles and the
conversion to nanoseconds is handled at the call site.

If the clock gets invalidated or vclock is already VCLOCK_NONE, return
U64_MAX as the cycle value, which is invalid for all clocks and leave the
sequence loop immediately in that case by calling the fallback function
directly.

This allows to remove the gettimeofday fallback as it now uses the
clock_gettime() fallback and does the nanoseconds to microseconds
conversion in the same way as it does when the vclock is functional. It
does not make a difference whether the division by 1000 happens in the
kernel fallback or in userspace.

Generates way better code and gains a few cycles back.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |   81 +++++++++--------------------------
 1 file changed, 21 insertions(+), 60 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -48,16 +48,6 @@ notrace static long vdso_fallback_gettim
 	return ret;
 }
 
-notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
-{
-	long ret;
-
-	asm("syscall" : "=a" (ret) :
-	    "0" (__NR_gettimeofday), "D" (tv), "S" (tz)
: "memory");
-	return ret;
-}
-
-
 #else
 
 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
@@ -75,21 +65,6 @@ notrace static long vdso_fallback_gettim
 	return ret;
 }
 
-notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
-{
-	long ret;
-
-	asm(
-		"mov %%ebx, %%edx \n"
-		"mov %2, %%ebx \n"
-		"call __kernel_vsyscall \n"
-		"mov %%edx, %%ebx \n"
-		: "=a" (ret)
-		: "0" (__NR_gettimeofday), "g" (tv), "c" (tz)
-		: "memory", "edx");
-	return ret;
-}
-
 #endif
 
 #ifdef CONFIG_PARAVIRT_CLOCK
@@ -98,7 +73,7 @@ static notrace const struct pvclock_vsys
 	return (const struct pvclock_vsyscall_time_info *)&pvclock_page;
 }
 
-static notrace u64 vread_pvclock(int *mode)
+static notrace u64 vread_pvclock(void)
 {
 	const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
 	u64 ret;
@@ -130,10 +105,8 @@ static notrace u64 vread_pvclock(int *mo
 	do {
 		version = pvclock_read_begin(pvti);
 
-		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
-			*mode = VCLOCK_NONE;
-			return 0;
-		}
+		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT)))
+			return U64_MAX;
 
 		ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
 	} while (pvclock_read_retry(pvti, version));
@@ -148,17 +121,12 @@ static notrace u64 vread_pvclock(int *mo
 }
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
-static notrace u64 vread_hvclock(int *mode)
+static notrace u64 vread_hvclock(void)
 {
 	const struct ms_hyperv_tsc_page *tsc_pg  		(const struct ms_hyperv_tsc_page
*)&hvclock_page;
-	u64 current_tick = hv_read_tsc_page(tsc_pg);
-
-	if (current_tick != U64_MAX)
-		return current_tick;
 
-	*mode = VCLOCK_NONE;
-	return 0;
+	return hv_read_tsc_page(tsc_pg);
 }
 #endif
 
@@ -182,47 +150,42 @@ notrace static u64 vread_tsc(void)
 	return last;
 }
 
-notrace static inline u64 vgetsns(int *mode)
+notrace static inline u64 vgetcyc(int mode)
 {
-	u64 v;
-	cycles_t cycles;
-
-	if (gtod->vclock_mode == VCLOCK_TSC)
-		cycles = vread_tsc();
+	if (mode == VCLOCK_TSC)
+		return vread_tsc();
 #ifdef CONFIG_PARAVIRT_CLOCK
-	else if (gtod->vclock_mode == VCLOCK_PVCLOCK)
-		cycles = vread_pvclock(mode);
+	else if (mode == VCLOCK_PVCLOCK)
+		return vread_pvclock();
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
-	else if (gtod->vclock_mode == VCLOCK_HVCLOCK)
-		cycles = vread_hvclock(mode);
+	else if (mode == VCLOCK_HVCLOCK)
+		return vread_hvclock();
 #endif
-	else
-		return 0;
-	v = cycles - gtod->cycle_last;
-	return v * gtod->mult;
+	return U64_MAX;
 }
 
 notrace static int do_hres(clockid_t clk, struct timespec *ts)
 {
 	struct vgtod_ts *base = &gtod->basetime[clk];
 	unsigned int seq;
-	int mode;
-	u64 ns;
+	u64 cycles, ns;
 
 	do {
 		seq = gtod_read_begin(gtod);
-		mode = gtod->vclock_mode;
 		ts->tv_sec = base->sec;
 		ns = base->nsec;
-		ns += vgetsns(&mode);
+		cycles = vgetcyc(gtod->vclock_mode);
+		if (unlikely((s64)cycles < 0))
+			return vdso_fallback_gettime(clk, ts);
+		ns += (cycles - gtod->cycle_last) * gtod->mult;
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
 	ts->tv_nsec = ns;
 
-	return mode;
+	return 0;
 }
 
 notrace static void do_coarse(clockid_t clk, struct timespec *ts)
@@ -251,8 +214,7 @@ notrace int __vdso_clock_gettime(clockid
 	 */
 	msk = 1U << clock;
 	if (likely(msk & VGTOD_HRES)) {
-		if (do_hres(clock, ts) != VCLOCK_NONE)
-			return 0;
+		return do_hres(clock, ts);
 	} else if (msk & VGTOD_COARSE) {
 		do_coarse(clock, ts);
 		return 0;
@@ -268,8 +230,7 @@ notrace int __vdso_gettimeofday(struct t
 	if (likely(tv != NULL)) {
 		struct timespec *ts = (struct timespec *) tv;
 
-		if (unlikely(do_hres(CLOCK_REALTIME, ts) == VCLOCK_NONE))
-			return vdso_fallback_gtod(tv, tz);
+		do_hres(CLOCK_REALTIME, ts);
 		tv->tv_usec /= 1000;
 	}
 	if (unlikely(tz != NULL)) {

Dereferencing gtod->cycle_last all over the place and foing the cycles <
last comparison in the vclock read functions generates horrible code. Doing
it at the call site is much better and gains a few cycles both for TSC and
pvclock.

Caveat: This adds the comparison to the hyperv vclock as well, but I have
no way to test that.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c |   39 ++++++-----------------------------
 1 file changed, 7 insertions(+), 32 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -76,9 +76,8 @@ static notrace const struct pvclock_vsys
 static notrace u64 vread_pvclock(void)
 {
 	const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
-	u64 ret;
-	u64 last;
 	u32 version;
+	u64 ret;
 
 	/*
 	 * Note: The kernel and hypervisor must guarantee that cpu ID
@@ -111,13 +110,7 @@ static notrace u64 vread_pvclock(void)
 		ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
 	} while (pvclock_read_retry(pvti, version));
 
-	/* refer to vread_tsc() comment for rationale */
-	last = gtod->cycle_last;
-
-	if (likely(ret >= last))
-		return ret;
-
-	return last;
+	return ret;
 }
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
@@ -130,30 +123,10 @@ static notrace u64 vread_hvclock(void)
 }
 #endif
 
-notrace static u64 vread_tsc(void)
-{
-	u64 ret = (u64)rdtsc_ordered();
-	u64 last = gtod->cycle_last;
-
-	if (likely(ret >= last))
-		return ret;
-
-	/*
-	 * GCC likes to generate cmov here, but this branch is extremely
-	 * predictable (it's just a function of time and the likely is
-	 * very likely) and there's a data dependence, so force GCC
-	 * to generate a branch instead.  I don't barrier() because
-	 * we don't actually need a barrier, and if this function
-	 * ever gets inlined it will generate worse code.
-	 */
-	asm volatile ("");
-	return last;
-}
-
 notrace static inline u64 vgetcyc(int mode)
 {
 	if (mode == VCLOCK_TSC)
-		return vread_tsc();
+		return (u64)rdtsc_ordered();
 #ifdef CONFIG_PARAVIRT_CLOCK
 	else if (mode == VCLOCK_PVCLOCK)
 		return vread_pvclock();
@@ -168,17 +141,19 @@ notrace static inline u64 vgetcyc(int mo
 notrace static int do_hres(clockid_t clk, struct timespec *ts)
 {
 	struct vgtod_ts *base = &gtod->basetime[clk];
+	u64 cycles, last, ns;
 	unsigned int seq;
-	u64 cycles, ns;
 
 	do {
 		seq = gtod_read_begin(gtod);
 		ts->tv_sec = base->sec;
 		ns = base->nsec;
+		last = gtod->cycle_last;
 		cycles = vgetcyc(gtod->vclock_mode);
 		if (unlikely((s64)cycles < 0))
 			return vdso_fallback_gettime(clk, ts);
-		ns += (cycles - gtod->cycle_last) * gtod->mult;
+		if (cycles > last)
+			ns += (cycles - last) * gtod->mult;
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));

With the storage array in place it's now trivial to support CLOCK_TAI in
the vdso. Instead of extending the array to accomodate CLOCK_TAI, make use
of the fact that:

 - CLOCK ids are set in stone
 - CLOCK_THREAD_CPUTIME is never going to be supported in the VDSO so
   the array slot 3 is unused
 - CLOCK_TAI is id 11 which results in 3 when masked with 0x3

Add the mask to the basetime array lookup and set up the CLOCK_TAI base
time in update_vsyscall().

The performance impact of the mask operation is within the noise.

Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
 arch/x86/entry/vdso/vclock_gettime.c    |    2 +-
 arch/x86/entry/vsyscall/vsyscall_gtod.c |    4 ++++
 arch/x86/include/asm/vgtod.h            |    6 +++++-
 3 files changed, 10 insertions(+), 2 deletions(-)

--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -140,7 +140,7 @@ notrace static inline u64 vgetcyc(int mo
 
 notrace static int do_hres(clockid_t clk, struct timespec *ts)
 {
-	struct vgtod_ts *base = &gtod->basetime[clk];
+	struct vgtod_ts *base = &gtod->basetime[clk & VGTOD_HRES_MASK];
 	u64 cycles, last, ns;
 	unsigned int seq;
 
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -51,6 +51,10 @@ void update_vsyscall(struct timekeeper *
 	base->sec = tk->xtime_sec;
 	base->nsec = tk->tkr_mono.xtime_nsec;
 
+	base = &vdata->basetime[VGTOD_TAI];
+	base->sec = tk->xtime_sec + (s64)tk->tai_offset;
+	base->nsec = tk->tkr_mono.xtime_nsec;
+
 	base = &vdata->basetime[CLOCK_MONOTONIC];
 	base->sec = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
 	nsec = tk->tkr_mono.xtime_nsec;
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -19,9 +19,13 @@ struct vgtod_ts {
 };
 
 #define VGTOD_BASES	(CLOCK_MONOTONIC_COARSE + 1)
-#define VGTOD_HRES	(BIT(CLOCK_REALTIME) | BIT(CLOCK_MONOTONIC))
+#define VGTOD_HRES	(BIT(CLOCK_REALTIME) | BIT(CLOCK_MONOTONIC) |
BIT(CLOCK_TAI))
 #define VGTOD_COARSE	(BIT(CLOCK_REALTIME_COARSE) | BIT(CLOCK_MONOTONIC_COARSE))
 
+/* Abuse CLOCK_THREAD_CPUTIME_ID for VGTOD CLOCK TAI */
+#define VGTOD_HRES_MASK	0x3
+#define VGTOD_TAI	(CLOCK_TAI & VGTOD_HRES_MASK)
+
 /*
  * vsyscall_gtod_data will be accessed by 32 and 64 bit code at the same time
  * so be carefull by modifying this structure.

> On Sep 14, 2018, at 5:50 AM, Thomas Gleixner <tglx at linutronix.de>
wrote:
> 
> With the storage array in place it's now trivial to support CLOCK_TAI
in
> the vdso. Instead of extending the array to accomodate CLOCK_TAI, make use
> of the fact that:
> 
> - CLOCK ids are set in stone
> - CLOCK_THREAD_CPUTIME is never going to be supported in the VDSO so
>   the array slot 3 is unused
> - CLOCK_TAI is id 11 which results in 3 when masked with 0x3
> 
> Add the mask to the basetime array lookup and set up the CLOCK_TAI base
> time in update_vsyscall().
That?s... horrible. In an amazing way. Can you add BUILD_BUG_ON somewhere to
assert that this actually works?
> 
> The performance impact of the mask operation is within the noise.
> 
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> ---
> arch/x86/entry/vdso/vclock_gettime.c    |    2 +-
> arch/x86/entry/vsyscall/vsyscall_gtod.c |    4 ++++
> arch/x86/include/asm/vgtod.h            |    6 +++++-
> 3 files changed, 10 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/entry/vdso/vclock_gettime.c
> +++ b/arch/x86/entry/vdso/vclock_gettime.c
> @@ -140,7 +140,7 @@ notrace static inline u64 vgetcyc(int mo
> 
> notrace static int do_hres(clockid_t clk, struct timespec *ts)
> {
> -    struct vgtod_ts *base = &gtod->basetime[clk];
> +    struct vgtod_ts *base = &gtod->basetime[clk &
VGTOD_HRES_MASK];
>    u64 cycles, last, ns;
>    unsigned int seq;
> 
> --- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
> @@ -51,6 +51,10 @@ void update_vsyscall(struct timekeeper *
>    base->sec = tk->xtime_sec;
>    base->nsec = tk->tkr_mono.xtime_nsec;
> 
> +    base = &vdata->basetime[VGTOD_TAI];
> +    base->sec = tk->xtime_sec + (s64)tk->tai_offset;
> +    base->nsec = tk->tkr_mono.xtime_nsec;
> +
>    base = &vdata->basetime[CLOCK_MONOTONIC];
>    base->sec = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
>    nsec = tk->tkr_mono.xtime_nsec;
> --- a/arch/x86/include/asm/vgtod.h
> +++ b/arch/x86/include/asm/vgtod.h
> @@ -19,9 +19,13 @@ struct vgtod_ts {
> };
> 
> #define VGTOD_BASES    (CLOCK_MONOTONIC_COARSE + 1)
> -#define VGTOD_HRES    (BIT(CLOCK_REALTIME) | BIT(CLOCK_MONOTONIC))
> +#define VGTOD_HRES    (BIT(CLOCK_REALTIME) | BIT(CLOCK_MONOTONIC) |
BIT(CLOCK_TAI))
> #define VGTOD_COARSE    (BIT(CLOCK_REALTIME_COARSE) |
BIT(CLOCK_MONOTONIC_COARSE))
> 
> +/* Abuse CLOCK_THREAD_CPUTIME_ID for VGTOD CLOCK TAI */
> +#define VGTOD_HRES_MASK    0x3
> +#define VGTOD_TAI    (CLOCK_TAI & VGTOD_HRES_MASK)
> +
> /*
>  * vsyscall_gtod_data will be accessed by 32 and 64 bit code at the same
time
>  * so be carefull by modifying this structure.
> 
>

On Fri, Sep 14, 2018 at 2:52 PM Thomas Gleixner <tglx at linutronix.de>
wrote:
>
> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> implementation, which extended the clockid switch case and added yet
> another slightly different copy of the same code.
>
> Especially the extended switch case is problematic as the compiler tends to
> generate a jump table which then requires to use retpolines. If jump tables
> are disabled it adds yet another conditional to the existing maze.
>
> This series takes a different approach by consolidating the almost
> identical functions into one implementation for high resolution clocks and
> one for the coarse grained clock ids by storing the base data for each
> clock id in an array which is indexed by the clock id.
>
> This completely eliminates the switch case and allows further
> simplifications of the code base, which at the end all together gain a few
> cycles performance or at least stay on par with todays code. The resulting
> performance depends heavily on the micro architecture and the compiler.
No objections from my side, just a few general remarks: Deepa
and I have discussed the vdso in the past for 2038. I have started
a patch that I'll have to redo on top of yours, but that is fine, your
cleanup is only going to help here.

More generally speaking, Deepa said that she would like to see
some generalization on the vdso before adding the time64_t
based variants. Your series probably makes x86 diverge more
from the others, which makes it harder to consolidate them again,
but we might just as well use your new implementation to base the
generic one on, and just move the other ones over to that.

A couple of architectures (s390, ia64, riscv, powerpc, arm64)
implement the vdso as assembler code at the moment, so they
won't be as easy to consolidate (other than outright replacing all
the code).

The other five:
arch/x86/entry/vdso/vclock_gettime.c
arch/sparc/vdso/vclock_gettime.c
arch/nds32/kernel/vdso/gettimeofday.c
arch/mips/vdso/gettimeofday.c
arch/arm/vdso/vgettimeofday.c

are basically all minor variations of the same code base and could be
consolidated to some degree.
Any suggestions here? Should we plan to do that consolitdation based on
your new version, or just add clock_gettime64 in arm32 and x86-32, and then
be done with it? The other ones will obviously still be fast for 32-bit time_t
and will have a working non-vdso sys_clock_getttime64().

I also wonder about clock_getres(): half the architectures seem to implement
it in vdso, but notably arm32 and x86 don't, and I had not expected it to be
performance critical given that the result is easily cached in user space.

         Arnd

Thomas Gleixner <tglx at linutronix.de> writes:
> Dereferencing gtod->cycle_last all over the place and foing the cycles
<
> last comparison in the vclock read functions generates horrible code. Doing
> it at the call site is much better and gains a few cycles both for TSC and
> pvclock.
>
> Caveat: This adds the comparison to the hyperv vclock as well, but I have
> no way to test that.
While the change looks obviously correct for Hyper-V vclock too, by saying
that you encouraged me to give the whole series a shot. I did and turns
out Hyper-V vclock is broken: simple clock_gettime(CLOCK_REALTIME, )
test goes from 70 cpu cycles to 1000 (meaning that we're falling back to
syscall I guess). Bisecting now, stay tuned!

-- 
  Vitaly

Thomas Gleixner <tglx at linutronix.de> writes:
> Runtime validate the VCLOCK_MODE in clocksource::archdata and disable
> VCLOCK if invalid, which disables the VDSO but keeps the system running.
>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> ---
>  arch/x86/Kconfig       |    1 +
>  arch/x86/kernel/time.c |   16 ++++++++++++++++
>  2 files changed, 17 insertions(+)
>
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -48,6 +48,7 @@ config X86
>  	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
>  	select ANON_INODES
>  	select ARCH_CLOCKSOURCE_DATA
> +	select ARCH_CLOCKSOURCE_INIT
>  	select ARCH_DISCARD_MEMBLOCK
>  	select ARCH_HAS_ACPI_TABLE_UPGRADE	if ACPI
>  	select ARCH_HAS_DEBUG_VIRTUAL
> --- a/arch/x86/kernel/time.c
> +++ b/arch/x86/kernel/time.c
> @@ -10,6 +10,7 @@
>   *
>   */
>
> +#include <linux/clocksource.h>
>  #include <linux/clockchips.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
> @@ -105,3 +106,18 @@ void __init time_init(void)
>  {
>  	late_time_init = x86_late_time_init;
>  }
> +
> +/*
> + * Sanity check the vdso related archdata content.
> + */
> +void clocksource_arch_init(struct clocksource *cs)
> +{
> +	if (cs->archdata.vclock_mode == VCLOCK_NONE)
> +		return;
> +
> +	if (cs->archdata.vclock_mode >= VCLOCK_MAX) {
It should be '>' as VCLOCK_MAX == VCLOCK_HVCLOCK == 3
> +		pr_warn("clocksource %s registered with invalid vclock_mode %d.
Disabling vclock.\n",
> +			cs->name, cs->archdata.vclock_mode);
> +		cs->archdata.vclock_mode = VCLOCK_NONE;
> +	}
> +}
-- 
  Vitaly

On Fri, 14 Sep 2018, Arnd Bergmann wrote:
> On Fri, Sep 14, 2018 at 2:52 PM Thomas Gleixner <tglx at
linutronix.de> wrote:
> A couple of architectures (s390, ia64, riscv, powerpc, arm64)
> implement the vdso as assembler code at the moment, so they
> won't be as easy to consolidate (other than outright replacing all
> the code).
> 
> The other five:
> arch/x86/entry/vdso/vclock_gettime.c
> arch/sparc/vdso/vclock_gettime.c
> arch/nds32/kernel/vdso/gettimeofday.c
> arch/mips/vdso/gettimeofday.c
> arch/arm/vdso/vgettimeofday.c
>
> are basically all minor variations of the same code base and could be
> consolidated to some degree.
> Any suggestions here? Should we plan to do that consolitdation based on
> your new version, or just add clock_gettime64 in arm32 and x86-32, and then
> be done with it? The other ones will obviously still be fast for 32-bit
time_t
> and will have a working non-vdso sys_clock_getttime64().
In principle consolidating all those implementations should be possible to
some extent and probably worthwhile. What's arch specific are the actual
accessors to the hardware clocks.
> I also wonder about clock_getres(): half the architectures seem to
implement
> it in vdso, but notably arm32 and x86 don't, and I had not expected it
to be
> performance critical given that the result is easily cached in user space.
getres() is not really performance critical, but adding it does not create
a huge problem either.

Thanks,

	tglx

On Fri, Sep 14, 2018 at 5:50 AM, Thomas Gleixner <tglx at linutronix.de>
wrote:
> The code flow for the vclocks is convoluted as it requires the vclocks
> which can be invalidated separately from the vsyscall_gtod_data sequence to
> store the fact in a separate variable. That's inefficient.
>
>  notrace static int do_hres(clockid_t clk, struct timespec *ts)
>  {
>         struct vgtod_ts *base = &gtod->basetime[clk];
>         unsigned int seq;
> -       int mode;
> -       u64 ns;
> +       u64 cycles, ns;
>
>         do {
>                 seq = gtod_read_begin(gtod);
> -               mode = gtod->vclock_mode;
>                 ts->tv_sec = base->sec;
>                 ns = base->nsec;
> -               ns += vgetsns(&mode);
> +               cycles = vgetcyc(gtod->vclock_mode);
> +               if (unlikely((s64)cycles < 0))
> +                       return vdso_fallback_gettime(clk, ts);
i was contemplating this, and I would suggest one of two optimizations:

1. have all the helpers return a struct containing a u64 and a bool,
and use that bool.  The compiler should do okay.

2. Be sneaky.  Later in the series, you do:

                if (unlikely((s64)cycles < 0))
                        return vdso_fallback_gettime(clk, ts);
-               ns += (cycles - gtod->cycle_last) * gtod->mult;
+               if (cycles > last)
+                       ns += (cycles - last) * gtod->mult;

How about:

if (unlikely((s64)cycles <= last)) {
  if (cycles < 0) [or cycles == -1 or whatever]
    return vdso_fallback_gettime;
} else {
  ns += (cycles - last) * gtod->mult;
}

which should actually make this whole mess be essentially free.

Also, I'm not entirely convinced that this "last" thing is needed
at
all.  John, what's the scenario under which we need it?

--Andy

--Andy

On Mon, 17 Sep 2018, John Stultz wrote:
> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto at
kernel.org> wrote:
> > Also, I'm not entirely convinced that this "last" thing
is needed at
> > all.  John, what's the scenario under which we need it?
> 
> So my memory is probably a bit foggy, but I recall that as we
> accelerated gettimeofday, we found that even on systems that claimed
> to have synced TSCs, they were actually just slightly out of sync.
> Enough that right after cycles_last had been updated, a read on
> another cpu could come in just behind cycles_last, resulting in a
> negative interval causing lots of havoc.
> 
> So the sanity check is needed to avoid that case.
Your memory serves you right. That's indeed observable on CPUs which
lack TSC_ADJUST.

@Andy: Welcome to the wonderful world of TSC.

Thanks,

	tglx

On Tue, Sep 18, 2018 at 09:52:26AM +0200, Thomas Gleixner
wrote:
> On Mon, 17 Sep 2018, John Stultz wrote:
> > On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto at
kernel.org> wrote:
> > > Also, I'm not entirely convinced that this "last"
thing is needed at
> > > all.  John, what's the scenario under which we need it?
> > 
> > So my memory is probably a bit foggy, but I recall that as we
> > accelerated gettimeofday, we found that even on systems that claimed
> > to have synced TSCs, they were actually just slightly out of sync.
> > Enough that right after cycles_last had been updated, a read on
> > another cpu could come in just behind cycles_last, resulting in a
> > negative interval causing lots of havoc.
> > 
> > So the sanity check is needed to avoid that case.
> 
> Your memory serves you right. That's indeed observable on CPUs which
> lack TSC_ADJUST.
But, if the gtod code can observe this, then why doesn't the code that
checks the sync?

> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx at linutronix.de>
wrote:
> 
>> On Mon, 17 Sep 2018, John Stultz wrote:
>>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto at
kernel.org> wrote:
>>> Also, I'm not entirely convinced that this "last"
thing is needed at
>>> all.  John, what's the scenario under which we need it?
>> 
>> So my memory is probably a bit foggy, but I recall that as we
>> accelerated gettimeofday, we found that even on systems that claimed
>> to have synced TSCs, they were actually just slightly out of sync.
>> Enough that right after cycles_last had been updated, a read on
>> another cpu could come in just behind cycles_last, resulting in a
>> negative interval causing lots of havoc.
>> 
>> So the sanity check is needed to avoid that case.
> 
> Your memory serves you right. That's indeed observable on CPUs which
> lack TSC_ADJUST.
> 
> @Andy: Welcome to the wonderful world of TSC.
> 
Do we do better if we use signed arithmetic for the whole calculation? Then a
small backwards movement would result in a small backwards result.  Or we could
offset everything so that we?d have to go back several hundred ms before we
cross zero.

Hi Vitaly, Paolo, Radim, etc.,

On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx at linutronix.de>
wrote:
>
> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> implementation, which extended the clockid switch case and added yet
> another slightly different copy of the same code.
>
> Especially the extended switch case is problematic as the compiler tends to
> generate a jump table which then requires to use retpolines. If jump tables
> are disabled it adds yet another conditional to the existing maze.
>
> This series takes a different approach by consolidating the almost
> identical functions into one implementation for high resolution clocks and
> one for the coarse grained clock ids by storing the base data for each
> clock id in an array which is indexed by the clock id.
>
I was trying to understand more of the implications of this patch
series, and I was again reminded that there is an entire extra copy of
the vclock reading code in arch/x86/kvm/x86.c.  And the purpose of
that code is very, very opaque.

Can one of you explain what the code is even doing?  From a couple of
attempts to read through it, it's a whole bunch of
probably-extremely-buggy code that, drumroll please, tries to
atomically read the TSC value and the time.  And decide whether the
result is "based on the TSC".  And then synthesizes a TSC-to-ns
multiplier and shift, based on *something other than the actual
multiply and shift used*.

IOW, unless I'm totally misunderstanding it, the code digs into the
private arch clocksource data intended for the vDSO, uses a poorly
maintained copy of the vDSO code to read the time (instead of doing
the sane thing and using the kernel interfaces for this), and
propagates a totally made up copy to the guest.  And gets it entirely
wrong when doing nested virt, since, unless there's some secret in
this maze, it doesn't acutlaly use the scaling factor from the host
when it tells the guest what to do.

I am really, seriously tempted to send a patch to simply delete all
this code.  The correct way to do it is to hook

And I don't see how it's even possible to pass kvmclock correctly to
the L2 guest when L0 is hyperv.  KVM could pass *hyperv's* clock, but
L1 isn't notified when the data structure changes, so how the heck is
it supposed to update the kvmclock structure?

--Andy

Andy Lutomirski <luto at kernel.org> writes:
> Hi Vitaly, Paolo, Radim, etc.,
>
> On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx at
linutronix.de> wrote:
>>
>> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
>> implementation, which extended the clockid switch case and added yet
>> another slightly different copy of the same code.
>>
>> Especially the extended switch case is problematic as the compiler
tends to
>> generate a jump table which then requires to use retpolines. If jump
tables
>> are disabled it adds yet another conditional to the existing maze.
>>
>> This series takes a different approach by consolidating the almost
>> identical functions into one implementation for high resolution clocks
and
>> one for the coarse grained clock ids by storing the base data for each
>> clock id in an array which is indexed by the clock id.
>>
>
> I was trying to understand more of the implications of this patch
> series, and I was again reminded that there is an entire extra copy of
> the vclock reading code in arch/x86/kvm/x86.c.  And the purpose of
> that code is very, very opaque.
>
> Can one of you explain what the code is even doing?  From a couple of
> attempts to read through it, it's a whole bunch of
> probably-extremely-buggy code that, drumroll please, tries to
> atomically read the TSC value and the time.  And decide whether the
> result is "based on the TSC".  And then synthesizes a TSC-to-ns
> multiplier and shift, based on *something other than the actual
> multiply and shift used*.
>
> IOW, unless I'm totally misunderstanding it, the code digs into the
> private arch clocksource data intended for the vDSO, uses a poorly
> maintained copy of the vDSO code to read the time (instead of doing
> the sane thing and using the kernel interfaces for this), and
> propagates a totally made up copy to the guest.  And gets it entirely
> wrong when doing nested virt, since, unless there's some secret in
> this maze, it doesn't acutlaly use the scaling factor from the host
> when it tells the guest what to do.
>
> I am really, seriously tempted to send a patch to simply delete all
> this code.  The correct way to do it is to hook
"I have discovered a truly marvelous proof of this, which this margin is
too narrow to contain" :-)

There is a very long history of different (hardware) issues Marcelo was
fighting with and the current code is the survived Frankenstein. E.g. it
is very, very unclear what "catchup", "always catchup" and
masterclock-less mode in general are and if we still need them.

That said I'm all for simplification. I'm not sure if we still need to
care about buggy hardware though.
>
> And I don't see how it's even possible to pass kvmclock correctly
to
> the L2 guest when L0 is hyperv.  KVM could pass *hyperv's* clock, but
> L1 isn't notified when the data structure changes, so how the heck is
> it supposed to update the kvmclock structure?
Well, this kind of works in the the followin way:
L1's clocksource is 'tsc_page' which is, basically, a compliment to
TSC:
two numbers provided by L0: offset and scale and KVM was tought to treat
this clocksource as a good one (see b0c39dc68e3b "x86/kvm: Pass stable
clocksource to guests when running nested on Hyper-V").

The notification you're talking about exists, it is called
Reenligntenment, see 0092e4346f49 "x86/kvm: Support Hyper-V
reenlightenment"). When TSC page changes (and this only happens when L1
is migrated to a different host with a different TSC frequency and TSC
scaling is not supported by the CPU) we receive an interrupt in L1 (at
this moment all TSC accesses are emulated which guarantees the
correctness of the readings), pause all L2 guests, update their kvmclock
structures with new data (we already know the new TSC frequency) and
then tell L0 that we're done and it can stop emulating TSC accesses.

(Nothing like this exists for KVM-on-KVM, by the way, when L1's
clocksource is 'kvmclock' L2s won't get a stable kvmclock
clocksource.)

-- 
Vitaly

On Tue, Oct 02, 2018 at 10:15:49PM -0700, Andy Lutomirski
wrote:
> Hi Vitaly, Paolo, Radim, etc.,
> 
> On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx at
linutronix.de> wrote:
> >
> > Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> > implementation, which extended the clockid switch case and added yet
> > another slightly different copy of the same code.
> >
> > Especially the extended switch case is problematic as the compiler
tends to
> > generate a jump table which then requires to use retpolines. If jump
tables
> > are disabled it adds yet another conditional to the existing maze.
> >
> > This series takes a different approach by consolidating the almost
> > identical functions into one implementation for high resolution clocks
and
> > one for the coarse grained clock ids by storing the base data for each
> > clock id in an array which is indexed by the clock id.
> >
> 
> I was trying to understand more of the implications of this patch
> series, and I was again reminded that there is an entire extra copy of
> the vclock reading code in arch/x86/kvm/x86.c.  And the purpose of
> that code is very, very opaque.
> 
> Can one of you explain what the code is even doing?  From a couple of
> attempts to read through it, it's a whole bunch of
> probably-extremely-buggy code that, 
Yes, probably.
> drumroll please, tries to atomically read the TSC value and the time.  And
decide whether the
> result is "based on the TSC".  
I think "based on the TSC" refers to whether TSC clocksource is being
used.
> And then synthesizes a TSC-to-ns
> multiplier and shift, based on *something other than the actual
> multiply and shift used*.
> 
> IOW, unless I'm totally misunderstanding it, the code digs into the
> private arch clocksource data intended for the vDSO, uses a poorly
> maintained copy of the vDSO code to read the time (instead of doing
> the sane thing and using the kernel interfaces for this), and
> propagates a totally made up copy to the guest.
I posted kernel interfaces for this, and it was suggested to 
instead write a "in-kernel user of pvclock data".

If you can get kernel interfaces to replace that, go for it. I prefer
kernel interfaces as well.
>  And gets it entirely
> wrong when doing nested virt, since, unless there's some secret in
> this maze, it doesn't acutlaly use the scaling factor from the host
> when it tells the guest what to do.
> 
> I am really, seriously tempted to send a patch to simply delete all
> this code.  
If your patch which deletes the code gets the necessary features right,
sure, go for it.
> The correct way to do it is to hook
Can you expand on the correct way to do it?
> And I don't see how it's even possible to pass kvmclock correctly
to
> the L2 guest when L0 is hyperv.  KVM could pass *hyperv's* clock, but
> L1 isn't notified when the data structure changes, so how the heck is
> it supposed to update the kvmclock structure?
I don't parse your question.
> 
> --Andy

On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx at linutronix.de>
wrote:
>
> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> implementation, which extended the clockid switch case and added yet
> another slightly different copy of the same code.
>
> Especially the extended switch case is problematic as the compiler tends to
> generate a jump table which then requires to use retpolines. If jump tables
> are disabled it adds yet another conditional to the existing maze.
>
> This series takes a different approach by consolidating the almost
> identical functions into one implementation for high resolution clocks and
> one for the coarse grained clock ids by storing the base data for each
> clock id in an array which is indexed by the clock id.
>
> This completely eliminates the switch case and allows further
> simplifications of the code base, which at the end all together gain a few
> cycles performance or at least stay on par with todays code. The resulting
> performance depends heavily on the micro architecture and the compiler.
tglx, please consider this whole series Acked-by: Andy Lutomirski
<luto at kernel.org>

Please feel free to push it top tip:x86/vdso, as long as you pull in
tip/x86/urgent first.  Once it lands, I'll email out a couple of
follow-up patches.

--Andy