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Groups > linux.kernel > #1651748

[RFC][PATCH 3/4] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW

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

Commit 45a7905fc48f ("arm64: vdso: defer shifting of nanosecond
component of timespec") fixed sub-ns inaccuracies in our vDSO
clock_gettime implementation by deferring the right-shift of the
nanoseconds components until after the timespec addition, which
operates on left-shifted values. That worked nicely until
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.

Now that the core code 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. 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@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Daniel Mentz <danielmentz@google.com>
Reported-by: John Stultz <john.stultz@linaro.org>
Acked-by: Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[jstultz: minor whitespace tweak]
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 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

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[RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs John Stultz <john.stultz@linaro.org> - 2017-05-27 05:40 +0200
  [RFC][PATCH 4/4] time: Clean up CLOCK_MONOTONIC_RAW time handling John Stultz <john.stultz@linaro.org> - 2017-05-27 05:40 +0200
  [RFC][PATCH 2/4] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting John Stultz <john.stultz@linaro.org> - 2017-05-27 05:40 +0200
    Re: [RFC][PATCH 2/4] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond  accounting Ingo Molnar <mingo@kernel.org> - 2017-05-27 09:40 +0200
      Re: [RFC][PATCH 2/4] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting Daniel Mentz <danielmentz@google.com> - 2017-05-30 20:50 +0200
  [RFC][PATCH 1/4] time: Fix clock->read(clock) race around clocksource changes John Stultz <john.stultz@linaro.org> - 2017-05-27 05:40 +0200
    Re: [RFC][PATCH 1/4] time: Fix clock->read(clock) race around  clocksource changes Ingo Molnar <mingo@kernel.org> - 2017-05-27 09:40 +0200
  [RFC][PATCH 3/4] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC_RAW John Stultz <john.stultz@linaro.org> - 2017-05-27 06:20 +0200
    Re: [RFC][PATCH 3/4] arm64: vdso: Fix nsec handling for  CLOCK_MONOTONIC_RAW Will Deacon <will.deacon@arm.com> - 2017-05-30 11:40 +0200
  Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs Ingo Molnar <mingo@kernel.org> - 2017-05-27 09:40 +0200
    Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs John Stultz <john.stultz@linaro.org> - 2017-05-27 18:20 +0200
      Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs Ingo Molnar <mingo@kernel.org> - 2017-05-28 11:00 +0200
  Re: [RFC][PATCH 0/4] Fixes for two recently found timekeeping bugs Daniel Mentz <danielmentz@google.com> - 2017-05-31 02:20 +0200

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