[patch 08/11] x86/vdso: Replace the clockid switch case

Thomas Gleixner
Fri, 14 Sep 2018 12:50:14 -0000

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")));