Re: [PATCH 2/2] arm64:vdso: Remove ISB from gettimeofday.
From: Jon Masters
Date: Sat May 06 2017 - 12:29:17 EST
On 04/23/2017 07:47 PM, Andrew Pinski wrote:
> ISB is normally required before mrs CNTVCT if we want the
> mrs to completed after the loads. In this case it is not.
> As we are taking the difference and if that difference
> was going to be negative, we just use the last counter value
> Signed-off-by: Andrew Pinski <apinski@xxxxxxxxxx>
Humor me. Walk me through this?
> arch/arm64/kernel/vdso/gettimeofday.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
> diff --git a/arch/arm64/kernel/vdso/gettimeofday.c b/arch/arm64/kernel/vdso/gettimeofday.c
> index a0ab8b1..cf3235a 100644
> --- a/arch/arm64/kernel/vdso/gettimeofday.c
> +++ b/arch/arm64/kernel/vdso/gettimeofday.c
> @@ -117,10 +117,20 @@ static notrace u64 get_clock_shifted_nsec(u64 cycle_last, u64 mult)
> u64 res;
> /* Read the virtual counter. */
> - isb();
> + /*
> + * This normally requires an ISB but since we know the
> + * read of the last cycle will always be after the
> + * read of the values are valid word.
> + */
> asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
> - res = res - cycle_last;
> + /*
> + * If the current cycle is greater than the last,
> + * then get the difference.
> + */
> + if (res > cycle_last)
> + res = res - cycle_last;
> /* We can only guarantee 56 bits of precision. */
> res &= ~(0xff00ul<<48);
> return res * mult;