Re: [PATCH v1 3/3] arm64: vdso: use SYS_CNTVCTSS_EL0 for gettimeofday

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Aug 25, 2022 at 11:20:25AM +0100, Joey Gouly wrote:
> If FEAT_ECV is implemented, the self-synchronized counter CNTVCTSS_EL0 can
> be used, removing the need for an ISB.
> 
> Signed-off-by: Joey Gouly <joey.gouly@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Andre Przywara <andre.przywara@arm.com>
> ---
>  arch/arm64/include/asm/vdso/gettimeofday.h | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
> index 4f7a629df81f..d8aff747019c 100644
> --- a/arch/arm64/include/asm/vdso/gettimeofday.h
> +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
> @@ -9,6 +9,8 @@
>  
>  #include <asm/barrier.h>
>  #include <asm/unistd.h>
> +#include <asm/sysreg.h>
> +#include <asm/alternative.h>

Nit: please sort this alphabetically.

>  
>  #define VDSO_HAS_CLOCK_GETRES		1
>  
> @@ -78,11 +80,15 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
>  		return 0;
>  
>  	/*
> -	 * This isb() is required to prevent that the counter value
> +	 * If FEAT_ECV is available, use the self-synchronizing counter.
> +	 * Otherwise the isb is required to prevent that the counter value
>  	 * is speculated.
> -	 */
> -	isb();
> -	asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
> +	*/
> +	asm volatile(ALTERNATIVE("isb\nmrs %0, cntvct_el0",
> +			"nop\n" __mrs_s("%0", SYS_CNTVCTSS_EL0),
> +			ARM64_HAS_ECV)
> +		     : "=r" (res) :: "memory");
> +

For legibility (and ease of future modification), could we please format this
with the fields all aligned, e.g.

	asm volatile(
	ALTERNATIVE("isb\n"
		    "mrs %0, cntvct_el0",
		    "nop\n"
		    __mrs_s("%0", SYS_CNTVCTSS_EL0),
		    ARM64_HAS_ECV)
	: "=r" (res)
	:
	: "memory");


Regardless:

	Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.

>  	arch_counter_enforce_ordering(res);
>  
>  	return res;
> -- 
> 2.17.1
> 

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