linux-kernel - [PATCH 3/3 v2] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC

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Date:   Wed, 31 May 2017 20:07:58 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     lkml <linux-kernel@...r.kernel.org>
Cc:     Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Daniel Mentz <danielmentz@...gle.com>,
        "stable #4 . 8+" <stable@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>
Subject: [PATCH 3/3 v2] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW

From: Will Deacon <will.deacon@....com>

Recently vDSO support for CLOCK_MONOTONIC_RAW was added in
49eea433b326 ("arm64: Add support for CLOCK_MONOTONIC_RAW in
clock_gettime() vDSO"). Noticing that the core timekeeping code
never set tkr_raw.xtime_nsec, the vDSO implementation didn't
bother exposing it via the data page and instead took the
unshifted tk->raw_time.tv_nsec value which was then immediately
shifted left in the vDSO code.

Unfortunately, by accellerating the MONOTONIC_RAW clockid, it
uncovered potential 1ns time inconsistencies caused by the
timekeeping core not handing sub-ns resolution.

Now that the core code has been fixed and is actually setting
tkr_raw.xtime_nsec, we need to take that into account in the
vDSO by adding it to the shifted raw_time value, in order to
fix the user-visible inconsistency. Rather than do that at each
use (and expand the data page in the process), instead perform
the shift/addition operation when populating the data page and
remove the shift from the vDSO code entirely.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Miroslav Lichvar <mlichvar@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Stephen Boyd <stephen.boyd@...aro.org>
Cc: Kevin Brodsky <kevin.brodsky@....com>
Cc: Will Deacon <will.deacon@....com>
Cc: Daniel Mentz <danielmentz@...gle.com>
Cc: stable <stable@...r.kernel.org> #4.8+
Reported-by: John Stultz <john.stultz@...aro.org>
Acked-by: Kevin Brodsky <kevin.brodsky@....com>
Tested-by: Daniel Mentz <danielmentz@...gle.com>
Signed-off-by: Will Deacon <will.deacon@....com>
[jstultz: minor whitespace tweak, tried to improve commit
 message to make it more clear this fixes a regression]
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
v2: Tweak commit message to address Ingo's feedback
---
 arch/arm64/kernel/vdso.c              | 5 +++--
 arch/arm64/kernel/vdso/gettimeofday.S | 1 -
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 41b6e31..d0cb007 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -221,10 +221,11 @@ void update_vsyscall(struct timekeeper *tk)
 		/* tkr_mono.cycle_last == tkr_raw.cycle_last */
 		vdso_data->cs_cycle_last	= tk->tkr_mono.cycle_last;
 		vdso_data->raw_time_sec		= tk->raw_time.tv_sec;
-		vdso_data->raw_time_nsec	= tk->raw_time.tv_nsec;
+		vdso_data->raw_time_nsec	= (tk->raw_time.tv_nsec <<
+						   tk->tkr_raw.shift) +
+						  tk->tkr_raw.xtime_nsec;
 		vdso_data->xtime_clock_sec	= tk->xtime_sec;
 		vdso_data->xtime_clock_nsec	= tk->tkr_mono.xtime_nsec;
-		/* tkr_raw.xtime_nsec == 0 */
 		vdso_data->cs_mono_mult		= tk->tkr_mono.mult;
 		vdso_data->cs_raw_mult		= tk->tkr_raw.mult;
 		/* tkr_mono.shift == tkr_raw.shift */
diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
index e00b467..76320e9 100644
--- a/arch/arm64/kernel/vdso/gettimeofday.S
+++ b/arch/arm64/kernel/vdso/gettimeofday.S
@@ -256,7 +256,6 @@ monotonic_raw:
 	seqcnt_check fail=monotonic_raw
 
 	/* All computations are done with left-shifted nsecs. */
-	lsl	x14, x14, x12
 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12
 
-- 
2.7.4