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Commit 45a7905f authored by Will Deacon's avatar Will Deacon Committed by Catalin Marinas
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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: default avatarJohn Stultz <john.stultz@linaro.org>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent d91fb5c2

arch/arm64/kernel/vdso.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

arch/arm64/kernel/vdso/gettimeofday.S

+6 −2
Original line number Diff line number Diff line
@@ -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