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Vitaly Kuznetsov

2017-Feb-14 12:44 UTC

[PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

Hi,

while we're still waiting for a definitive ACK from Microsoft that the
algorithm is good for SMP case (as we can't prevent the code in vdso from
migrating between CPUs) I'd like to send v2 with some modifications to keep
the discussion going.

Changes since v1:
- Document the TSC page reading protocol [Thomas Gleixner].

- Separate the TSC page reading code from read_hv_clock_tsc() and put it to
  asm/mshyperv.h to use from both hv_init.c and vdso.

- Add explicit barriers [Thomas Gleixner]

Original description:

Hyper-V TSC page clocksource is suitable for vDSO, however, the protocol
defined by the hypervisor is different from VCLOCK_PVCLOCK. Implemented the
required support. Simple sysbench test shows the following results:

Before:
# time sysbench --test=memory --max-requests=500000 run
...
real    1m22.618s
user    0m50.193s
sys     0m32.268s

After:
# time sysbench --test=memory --max-requests=500000 run
...
real	0m47.241s
user	0m47.117s
sys	0m0.008s

Patches 1 and 2 are made on top of K. Y.'s code refactoring which moved tsc
page clocksource to arch/x86/hyperv/hv_init.c, this is currently present in
Greg's char-misc-next tree.

Vitaly Kuznetsov (3):
  x86/hyperv: implement hv_get_tsc_page()
  x86/hyperv: move TSC reading method to asm/mshyperv.h
  x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

 arch/x86/entry/vdso/vclock_gettime.c  | 24 +++++++++++++++
 arch/x86/entry/vdso/vdso-layout.lds.S |  3 +-
 arch/x86/entry/vdso/vdso2c.c          |  3 ++
 arch/x86/entry/vdso/vma.c             |  7 +++++
 arch/x86/hyperv/hv_init.c             | 48 +++++++++--------------------
 arch/x86/include/asm/clocksource.h    |  3 +-
 arch/x86/include/asm/mshyperv.h       | 58 +++++++++++++++++++++++++++++++++++
 arch/x86/include/asm/vdso.h           |  1 +
 drivers/hv/Kconfig                    |  3 ++
 9 files changed, 114 insertions(+), 36 deletions(-)

-- 
2.9.3

Vitaly Kuznetsov

2017-Feb-14 12:44 UTC

head link

[PATCH v2 1/3] x86/hyperv: implement hv_get_tsc_page()

To use Hyper-V TSC page clocksource from vDSO we need to make tsc_pg
available. Implement hv_get_tsc_page() and add CONFIG_HYPERV_TSCPAGE to
make #ifdef-s simple.

Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
---
 arch/x86/hyperv/hv_init.c       | 9 +++++++--
 arch/x86/include/asm/mshyperv.h | 8 ++++++++
 drivers/hv/Kconfig              | 3 +++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index b371d0e..0ce8485 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -27,10 +27,15 @@
 #include <linux/clockchips.h>
 
 
-#ifdef CONFIG_X86_64
+#ifdef CONFIG_HYPERV_TSCPAGE
 
 static struct ms_hyperv_tsc_page *tsc_pg;
 
+struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
+{
+	return tsc_pg;
+}
+
 static u64 read_hv_clock_tsc(struct clocksource *arg)
 {
 	u64 current_tick;
@@ -136,7 +141,7 @@ void hyperv_init(void)
 	/*
 	 * Register Hyper-V specific clocksource.
 	 */
-#ifdef CONFIG_X86_64
+#ifdef CONFIG_HYPERV_TSCPAGE
 	if (ms_hyperv.features & HV_X64_MSR_REFERENCE_TSC_AVAILABLE) {
 		union hv_x64_msr_hypercall_contents tsc_msr;
 
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index f8dc370..14dd92c 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -173,4 +173,12 @@ void hyperv_report_panic(struct pt_regs *regs);
 bool hv_is_hypercall_page_setup(void);
 void hyperv_cleanup(void);
 #endif
+#ifdef CONFIG_HYPERV_TSCPAGE
+struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
+#else
+static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
+{
+	return NULL;
+}
+#endif
 #endif
diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
index 0403b51..c29cd53 100644
--- a/drivers/hv/Kconfig
+++ b/drivers/hv/Kconfig
@@ -7,6 +7,9 @@ config HYPERV
 	  Select this option to run Linux as a Hyper-V client operating
 	  system.
 
+config HYPERV_TSCPAGE
+       def_bool HYPERV && X86_64
+
 config HYPERV_UTILS
 	tristate "Microsoft Hyper-V Utilities driver"
 	depends on HYPERV && CONNECTOR && NLS
-- 
2.9.3

Vitaly Kuznetsov

2017-Feb-14 12:44 UTC

head link

[PATCH v2 2/3] x86/hyperv: move TSC reading method to asm/mshyperv.h

As a preparation to making Hyper-V TSC page suitable for vDSO move
the TSC page reading logic to asm/mshyperv.h. While on it, do the
following
- Document the reading algorithm.
- Simplify the code a bit.
- Add explicit barriers to prevent re-ordering (we need to read sequence
  stricktly before and after)
- Use mul_u64_u64_shr() instead of assembly. I checked and on x86_64
  gcc generates a single 'mul' instruction.

Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
---
 arch/x86/hyperv/hv_init.c       | 36 ++++-------------------------
 arch/x86/include/asm/mshyperv.h | 50 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 32 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 0ce8485..77fec0c 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -38,39 +38,11 @@ struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
 
 static u64 read_hv_clock_tsc(struct clocksource *arg)
 {
-	u64 current_tick;
+	u64 current_tick = hv_read_tsc_page(tsc_pg);
+
+	if (current_tick == U64_MAX)
+		rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);
 
-	if (tsc_pg->tsc_sequence != 0) {
-		/*
-		 * Use the tsc page to compute the value.
-		 */
-
-		while (1) {
-			u64 tmp;
-			u32 sequence = tsc_pg->tsc_sequence;
-			u64 cur_tsc;
-			u64 scale = tsc_pg->tsc_scale;
-			s64 offset = tsc_pg->tsc_offset;
-
-			rdtscll(cur_tsc);
-			/* current_tick = ((cur_tsc *scale) >> 64) + offset */
-			asm("mulq %3"
-				: "=d" (current_tick), "=a" (tmp)
-				: "a" (cur_tsc), "r" (scale));
-
-			current_tick += offset;
-			if (tsc_pg->tsc_sequence == sequence)
-				return current_tick;
-
-			if (tsc_pg->tsc_sequence != 0)
-				continue;
-			/*
-			 * Fallback using MSR method.
-			 */
-			break;
-		}
-	}
-	rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);
 	return current_tick;
 }
 
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 14dd92c..ddd071c 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -175,6 +175,56 @@ void hyperv_cleanup(void);
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
 struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
+static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
+{
+	u64 scale, offset, current_tick, cur_tsc;
+	u32 sequence;
+
+	/*
+	 * The protocol for reading Hyper-V TSC page is specified in Hypervisor
+	 * Top-Level Functional Specification ver. 3.0 and above. To get the
+	 * reference time we must do the following:
+	 * - READ ReferenceTscSequence
+	 *   A special '0' value indicates the time source is unreliable and
we
+	 *   need to use something else. The currently published specification
+	 *   versions (up to 4.0b) contain a mistake and wrongly claim '-1'
+	 *   instead of '0' as the special value, see commit c35b82ef0294.
+	 * - ReferenceTime +	 *        ((RDTSC() * ReferenceTscScale) >> 64) +
ReferenceTscOffset
+	 * - READ ReferenceTscSequence again. In case its value has changed
+	 *   since our first reading we need to discard ReferenceTime and repeat
+	 *   the whole sequence as the hypervisor was updating the page in
+	 *   between.
+	 */
+	while (1) {
+		sequence = tsc_pg->tsc_sequence;
+		if (!sequence)
+			break;
+		/*
+		 * Make sure we read sequence before we read other values from
+		 * TSC page.
+		 */
+		virt_rmb();
+
+		scale = tsc_pg->tsc_scale;
+		offset = tsc_pg->tsc_offset;
+		rdtscll(cur_tsc);
+
+		current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
+
+		/*
+		 * Make sure we read sequence after we read all other values
+		 * from TSC page.
+		 */
+		virt_rmb();
+
+		if (tsc_pg->tsc_sequence == sequence)
+			return current_tick;
+	}
+
+	return U64_MAX;
+}
+
 #else
 static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
 {
-- 
2.9.3

Vitaly Kuznetsov

2017-Feb-14 12:44 UTC

head link

[PATCH v2 3/3] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

Hyper-V TSC page clocksource is suitable for vDSO, however, the protocol
defined by the hypervisor is different from VCLOCK_PVCLOCK. Implement the
required support by adding hvclock_page VVAR.

Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
---
 arch/x86/entry/vdso/vclock_gettime.c  | 24 ++++++++++++++++++++++++
 arch/x86/entry/vdso/vdso-layout.lds.S |  3 ++-
 arch/x86/entry/vdso/vdso2c.c          |  3 +++
 arch/x86/entry/vdso/vma.c             |  7 +++++++
 arch/x86/hyperv/hv_init.c             |  3 +++
 arch/x86/include/asm/clocksource.h    |  3 ++-
 arch/x86/include/asm/vdso.h           |  1 +
 7 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c
b/arch/x86/entry/vdso/vclock_gettime.c
index 9d4d6e1..fa8dbfc 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -17,6 +17,7 @@
 #include <asm/unistd.h>
 #include <asm/msr.h>
 #include <asm/pvclock.h>
+#include <asm/mshyperv.h>
 #include <linux/math64.h>
 #include <linux/time.h>
 #include <linux/kernel.h>
@@ -32,6 +33,11 @@ extern u8 pvclock_page
 	__attribute__((visibility("hidden")));
 #endif
 
+#ifdef CONFIG_HYPERV_TSCPAGE
+extern u8 hvclock_page
+	__attribute__((visibility("hidden")));
+#endif
+
 #ifndef BUILD_VDSO32
 
 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
@@ -141,6 +147,20 @@ static notrace u64 vread_pvclock(int *mode)
 	return last;
 }
 #endif
+#ifdef CONFIG_HYPERV_TSCPAGE
+static notrace u64 vread_hvclock(int *mode)
+{
+	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;
+}
+#endif
 
 notrace static u64 vread_tsc(void)
 {
@@ -173,6 +193,10 @@ notrace static inline u64 vgetsns(int *mode)
 	else if (gtod->vclock_mode == VCLOCK_PVCLOCK)
 		cycles = vread_pvclock(mode);
 #endif
+#ifdef CONFIG_HYPERV_TSCPAGE
+	else if (gtod->vclock_mode == VCLOCK_HVCLOCK)
+		cycles = vread_hvclock(mode);
+#endif
 	else
 		return 0;
 	v = (cycles - gtod->cycle_last) & gtod->mask;
diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S
b/arch/x86/entry/vdso/vdso-layout.lds.S
index a708aa9..8ebb4b6 100644
--- a/arch/x86/entry/vdso/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/vdso-layout.lds.S
@@ -25,7 +25,7 @@ SECTIONS
 	 * segment.
 	 */
 
-	vvar_start = . - 2 * PAGE_SIZE;
+	vvar_start = . - 3 * PAGE_SIZE;
 	vvar_page = vvar_start;
 
 	/* Place all vvars at the offsets in asm/vvar.h. */
@@ -36,6 +36,7 @@ SECTIONS
 #undef EMIT_VVAR
 
 	pvclock_page = vvar_start + PAGE_SIZE;
+	hvclock_page = vvar_start + 2 * PAGE_SIZE;
 
 	. = SIZEOF_HEADERS;
 
diff --git a/arch/x86/entry/vdso/vdso2c.c b/arch/x86/entry/vdso/vdso2c.c
index 491020b..0780a44 100644
--- a/arch/x86/entry/vdso/vdso2c.c
+++ b/arch/x86/entry/vdso/vdso2c.c
@@ -74,6 +74,7 @@ enum {
 	sym_vvar_page,
 	sym_hpet_page,
 	sym_pvclock_page,
+	sym_hvclock_page,
 	sym_VDSO_FAKE_SECTION_TABLE_START,
 	sym_VDSO_FAKE_SECTION_TABLE_END,
 };
@@ -82,6 +83,7 @@ const int special_pages[] = {
 	sym_vvar_page,
 	sym_hpet_page,
 	sym_pvclock_page,
+	sym_hvclock_page,
 };
 
 struct vdso_sym {
@@ -94,6 +96,7 @@ struct vdso_sym required_syms[] = {
 	[sym_vvar_page] = {"vvar_page", true},
 	[sym_hpet_page] = {"hpet_page", true},
 	[sym_pvclock_page] = {"pvclock_page", true},
+	[sym_hvclock_page] = {"hvclock_page", true},
 	[sym_VDSO_FAKE_SECTION_TABLE_START] = {
 		"VDSO_FAKE_SECTION_TABLE_START", false
 	},
diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index 10820f6..b256a3b 100644
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -21,6 +21,7 @@
 #include <asm/page.h>
 #include <asm/desc.h>
 #include <asm/cpufeature.h>
+#include <asm/mshyperv.h>
 
 #if defined(CONFIG_X86_64)
 unsigned int __read_mostly vdso64_enabled = 1;
@@ -120,6 +121,12 @@ static int vvar_fault(const struct vm_special_mapping *sm,
 				vmf->address,
 				__pa(pvti) >> PAGE_SHIFT);
 		}
+	} else if (sym_offset == image->sym_hvclock_page) {
+		struct ms_hyperv_tsc_page *tsc_pg = hv_get_tsc_page();
+
+		if (tsc_pg && vclock_was_used(VCLOCK_HVCLOCK))
+			ret = vm_insert_pfn(vma, vmf->address,
+					    vmalloc_to_pfn(tsc_pg));
 	}
 
 	if (ret == 0 || ret == -EBUSY)
diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 77fec0c..870f0eb 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -129,6 +129,9 @@ void hyperv_init(void)
 		tsc_msr.guest_physical_address = vmalloc_to_pfn(tsc_pg);
 
 		wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
+
+		hyperv_cs_tsc.archdata.vclock_mode = VCLOCK_HVCLOCK;
+
 		clocksource_register_hz(&hyperv_cs_tsc, NSEC_PER_SEC/100);
 		return;
 	}
diff --git a/arch/x86/include/asm/clocksource.h
b/arch/x86/include/asm/clocksource.h
index eae33c7..47bea8c 100644
--- a/arch/x86/include/asm/clocksource.h
+++ b/arch/x86/include/asm/clocksource.h
@@ -6,7 +6,8 @@
 #define VCLOCK_NONE	0	/* No vDSO clock available.		*/
 #define VCLOCK_TSC	1	/* vDSO should use vread_tsc.		*/
 #define VCLOCK_PVCLOCK	2	/* vDSO should use vread_pvclock.	*/
-#define VCLOCK_MAX	2
+#define VCLOCK_HVCLOCK	3	/* vDSO should use vread_hvclock.	*/
+#define VCLOCK_MAX	3
 
 struct arch_clocksource_data {
 	int vclock_mode;
diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h
index 2444189..bccdf49 100644
--- a/arch/x86/include/asm/vdso.h
+++ b/arch/x86/include/asm/vdso.h
@@ -20,6 +20,7 @@ struct vdso_image {
 	long sym_vvar_page;
 	long sym_hpet_page;
 	long sym_pvclock_page;
+	long sym_hvclock_page;
 	long sym_VDSO32_NOTE_MASK;
 	long sym___kernel_sigreturn;
 	long sym___kernel_rt_sigreturn;
-- 
2.9.3

KY Srinivasan

2017-Feb-14 14:46 UTC

head link

[PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets at redhat.com]
> Sent: Tuesday, February 14, 2017 4:44 AM
> To: x86 at kernel.org; Andy Lutomirski <luto at amacapital.net>
> Cc: Thomas Gleixner <tglx at linutronix.de>; Ingo Molnar <mingo at
redhat.com>;
> H. Peter Anvin <hpa at zytor.com>; KY Srinivasan <kys at
microsoft.com>;
> Haiyang Zhang <haiyangz at microsoft.com>; Stephen Hemminger
> <sthemmin at microsoft.com>; Dexuan Cui <decui at
microsoft.com>; linux-
> kernel at vger.kernel.org; devel at linuxdriverproject.org;
> virtualization at lists.linux-foundation.org
> Subject: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support
> 
> Hi,
> 
> while we're still waiting for a definitive ACK from Microsoft that the
algorithm
> is good for SMP case (as we can't prevent the code in vdso from
migrating
> between CPUs) I'd like to send v2 with some modifications to keep the
> discussion going.
I checked with the folks on the Hyper-V side and they have confirmed that we
need to
add memory barriers in the guest code to ensure the various reads from the TSC
page are
correctly ordered - especially, the initial read of the sequence counter must
have acquire
semantics. We should ensure that other reads from the TSC page are completed
before the
second read of the sequence counter. I am working with the Windows team to
correctly
reflect this algorithm in the Hyper-V specification.

Regards,

K. Y> 
> Changes since v1:
> - Document the TSC page reading protocol [Thomas Gleixner].
> 
> - Separate the TSC page reading code from read_hv_clock_tsc() and put it to
>   asm/mshyperv.h to use from both hv_init.c and vdso.
> 
> - Add explicit barriers [Thomas Gleixner]
> 
> Original description:
> 
> Hyper-V TSC page clocksource is suitable for vDSO, however, the protocol
> defined by the hypervisor is different from VCLOCK_PVCLOCK. Implemented
> the required support. Simple sysbench test shows the following results:
> 
> Before:
> # time sysbench --test=memory --max-requests=500000 run ...
> real    1m22.618s
> user    0m50.193s
> sys     0m32.268s
> 
> After:
> # time sysbench --test=memory --max-requests=500000 run ...
> real	0m47.241s
> user	0m47.117s
> sys	0m0.008s
> 
> Patches 1 and 2 are made on top of K. Y.'s code refactoring which moved
tsc
> page clocksource to arch/x86/hyperv/hv_init.c, this is currently present in
> Greg's char-misc-next tree.
> 
> Vitaly Kuznetsov (3):
>   x86/hyperv: implement hv_get_tsc_page()
>   x86/hyperv: move TSC reading method to asm/mshyperv.h
>   x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method
> 
>  arch/x86/entry/vdso/vclock_gettime.c  | 24 +++++++++++++++
> arch/x86/entry/vdso/vdso-layout.lds.S |  3 +-
>  arch/x86/entry/vdso/vdso2c.c          |  3 ++
>  arch/x86/entry/vdso/vma.c             |  7 +++++
>  arch/x86/hyperv/hv_init.c             | 48 +++++++++--------------------
>  arch/x86/include/asm/clocksource.h    |  3 +-
>  arch/x86/include/asm/mshyperv.h       | 58
> +++++++++++++++++++++++++++++++++++
>  arch/x86/include/asm/vdso.h           |  1 +
>  drivers/hv/Kconfig                    |  3 ++
>  9 files changed, 114 insertions(+), 36 deletions(-)
> 
> --
> 2.9.3

Thomas Gleixner

2017-Feb-14 14:56 UTC

head link

[PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

On Tue, 14 Feb 2017, Vitaly Kuznetsov wrote:
> Hi,
> 
> while we're still waiting for a definitive ACK from Microsoft that the
> algorithm is good for SMP case (as we can't prevent the code in vdso
from
> migrating between CPUs) I'd like to send v2 with some modifications to
keep
> the discussion going.
Migration is irrelevant. The TSC page is guest global so updates will
happen on some (random) host CPU and therefor you need the usual barriers
like we have them in our seqcounts unless an access to the sequence will
trap into the host, which would defeat the whole purpose of the TSC page.

Thanks,

	tglx

Vitaly Kuznetsov

2017-Feb-14 15:50 UTC

head link

[PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

Thomas Gleixner <tglx at linutronix.de> writes:
> On Tue, 14 Feb 2017, Vitaly Kuznetsov wrote:
>
>> Hi,
>> 
>> while we're still waiting for a definitive ACK from Microsoft that
the
>> algorithm is good for SMP case (as we can't prevent the code in
vdso from
>> migrating between CPUs) I'd like to send v2 with some modifications
to keep
>> the discussion going.
>
> Migration is irrelevant. The TSC page is guest global so updates will
> happen on some (random) host CPU and therefor you need the usual barriers
> like we have them in our seqcounts unless an access to the sequence will
> trap into the host, which would defeat the whole purpose of the TSC page.
>
KY Srinivasan <kys at microsoft.com> writes:
> I checked with the folks on the Hyper-V side and they have confirmed that
we need to
> add memory barriers in the guest code to ensure the various reads from the
TSC page are
> correctly ordered - especially, the initial read of the sequence counter
must have acquire
> semantics. We should ensure that other reads from the TSC page are
completed before the
> second read of the sequence counter. I am working with the Windows team to
correctly
> reflect this algorithm in the Hyper-V specification.

Thank you,

do I get it right that combining the above I only need to replace
virt_rmb() barriers with plain rmb() to get 'lfence' in hv_read_tsc_page
(PATCH 2)? As members of struct ms_hyperv_tsc_page are volatile we don't
need READ_ONCE(), compilers are not allowed to merge accesses. The
resulting code looks good to me:

(gdb) disassemble read_hv_clock_tsc 
Dump of assembler code for function read_hv_clock_tsc:
   0xffffffff8102ca60 <+0>:	callq  0xffffffff816c7500 <__fentry__>
   0xffffffff8102ca65 <+5>:	mov    0xf67974(%rip),%rcx        #
0xffffffff81f943e0 <tsc_pg>
   0xffffffff8102ca6c <+12>:	jmp    0xffffffff8102ca87
<read_hv_clock_tsc+39>
   0xffffffff8102ca6e <+14>:	lfence 
   0xffffffff8102ca71 <+17>:	mov    0x8(%rcx),%r9
   0xffffffff8102ca75 <+21>:	mov    0x10(%rcx),%r8
   0xffffffff8102ca79 <+25>:	nop
   0xffffffff8102ca7a <+26>:	nop
   0xffffffff8102ca7b <+27>:	nop
   0xffffffff8102ca7c <+28>:	rdtsc  
   0xffffffff8102ca7e <+30>:	lfence 
   0xffffffff8102ca81 <+33>:	mov    (%rcx),%edi
   0xffffffff8102ca83 <+35>:	cmp    %edi,%esi
   0xffffffff8102ca85 <+37>:	je     0xffffffff8102caa3
<read_hv_clock_tsc+67>
   0xffffffff8102ca87 <+39>:	mov    (%rcx),%esi
   0xffffffff8102ca89 <+41>:	test   %esi,%esi
   0xffffffff8102ca8b <+43>:	jne    0xffffffff8102ca6e
<read_hv_clock_tsc+14>
   0xffffffff8102ca8d <+45>:	push   %rbp
   0xffffffff8102ca8e <+46>:	mov    $0x40000020,%edi
   0xffffffff8102ca93 <+51>:	mov    %rsp,%rbp
   0xffffffff8102ca96 <+54>:	and    $0xfffffffffffffff0,%rsp
   0xffffffff8102ca9a <+58>:	callq  *0xffffffff81c36330
   0xffffffff8102caa1 <+65>:	leaveq 
   0xffffffff8102caa2 <+66>:	retq   
   0xffffffff8102caa3 <+67>:	shl    $0x20,%rdx
   0xffffffff8102caa7 <+71>:	or     %rdx,%rax
   0xffffffff8102caaa <+74>:	mul    %r9
   0xffffffff8102caad <+77>:	mov    %rdx,%rax
   0xffffffff8102cab0 <+80>:	add    %r8,%rax
   0xffffffff8102cab3 <+83>:	cmp    $0xffffffffffffffff,%rax
   0xffffffff8102cab7 <+87>:	je     0xffffffff8102ca8d
<read_hv_clock_tsc+45>
   0xffffffff8102cab9 <+89>:	repz retq 
End of assembler dump.

-- 
  Vitaly

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