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>

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index ba45794..c958cb8 100644 (file)
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -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;

diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
index 99b7d40..6681f40 100644 (file)
--- a/arch/arm64/kernel/vdso/gettimeofday.S
+++ b/arch/arm64/kernel/vdso/gettimeofday.S
@@ -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