Messages in this thread | | | Subject | Re: [PATCH 2/2] arm64:vdso: Remove ISB from gettimeofday. | From | Jon Masters <> | Date | Sat, 6 May 2017 12:29:03 -0400 |
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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 > instead. > > Signed-off-by: Andrew Pinski <apinski@cavium.com>
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; >
-- Computer Architect
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