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arm64: vdso: defer shifting of nanosecond component of timespec 

Shifting the nanosecond component of the computed timespec early can
lead to sub-ns inaccuracies when using the truncated value as input to
further arithmetic for things like conversions to monotonic time.

This patch defers the timespec shifting until after the final value has
been computed.

Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Will Deacon 2012-11-29 22:33:29 +00:00 committed by Catalin Marinas
parent d91fb5c267
commit 45a7905fc4
2 changed files with 7 additions and 3 deletions
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2
arch/arm64/kernel/vdso.c
View File

@ -239,7 +239,7 @@ void update_vsyscall(struct timekeeper *tk)
if (!use_syscall) {
vdso_data->cs_cycle_last = tk->clock->cycle_last;
vdso_data->xtime_clock_sec = tk->xtime_sec;
vdso_data->xtime_clock_nsec = tk->xtime_nsec >> tk->shift;
vdso_data->xtime_clock_nsec = tk->xtime_nsec;
vdso_data->cs_mult = tk->mult;
vdso_data->cs_shift = tk->shift;
vdso_data->wtm_clock_sec = tk->wall_to_monotonic.tv_sec;

8
arch/arm64/kernel/vdso/gettimeofday.S
View File

@ -66,6 +66,7 @@ ENTRY(__kernel_gettimeofday)
/* Convert ns to us. */
mov x13, #1000
lsl x13, x13, x12
udiv x11, x11, x13
stp x10, x11, [x0, #TVAL_TV_SEC]
2:
@ -136,11 +137,13 @@ ENTRY(__kernel_clock_gettime)
4:
/* Add on wtm timespec. */
add x10, x10, x13
lsl x14, x14, x12
add x11, x11, x14
/* Normalise the new timespec. */
mov x15, #NSEC_PER_SEC_LO16
movk x15, #NSEC_PER_SEC_HI16, lsl #16
lsl x15, x15, x12
cmp x11, x15
b.lt 5f
sub x11, x11, x15
@ -152,6 +155,7 @@ ENTRY(__kernel_clock_gettime)
sub x10, x10, #1
6: /* Store to the user timespec. */
lsr x11, x11, x12
stp x10, x11, [x1, #TSPEC_TV_SEC]
mov x0, xzr
ret x2
@ -204,7 +208,7 @@ ENDPROC(__kernel_clock_getres)
* Clobbers the temporary registers (x9 - x15).
* Returns:
* - w9 = vDSO sequence counter
* - (x10, x11) = (ts->tv_sec, ts->tv_nsec)
* - (x10, x11) = (ts->tv_sec, shifted ts->tv_nsec)
* - w12 = cs_shift
*/
ENTRY(__do_get_tspec)
@ -226,11 +230,11 @@ ENTRY(__do_get_tspec)
movn x15, #0xff00, lsl #48
and x10, x15, x10
mul x10, x10, x11
lsr x10, x10, x12
/* Use the kernel time to calculate the new timespec. */
mov x11, #NSEC_PER_SEC_LO16
movk x11, #NSEC_PER_SEC_HI16, lsl #16
lsl x11, x11, x12
add x15, x10, x14
udiv x14, x15, x11
add x10, x13, x14