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Subject[patch 08/11] x86/vdso: Replace the clockid switch case
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@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")));


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