[PATCH 4/5] arm64: vdso: Avoid ISB after reading from cntvct_el0

[Vincenzo Frascino]

On 3/18/21 5:07 PM, Will Deacon wrote:
> We can avoid the expensive ISB instruction after reading the counter in
> the vDSO gettime functions by creating a fake address hazard against a
> dummy stack read, just like we do inside the kernel.
> 
> Signed-off-by: Will Deacon <will at kernel.org>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino at arm.com>

> ---
>  arch/arm64/include/asm/arch_timer.h        | 21 ---------------------
>  arch/arm64/include/asm/barrier.h           | 19 +++++++++++++++++++
>  arch/arm64/include/asm/vdso/gettimeofday.h |  6 +-----
>  3 files changed, 20 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 9f0ec21d6327..88d20f04c64a 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -165,25 +165,6 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
>  	isb();
>  }
>  
> -/*
> - * Ensure that reads of the counter are treated the same as memory reads
> - * for the purposes of ordering by subsequent memory barriers.
> - *
> - * This insanity brought to you by speculative system register reads,
> - * out-of-order memory accesses, sequence locks and Thomas Gleixner.
> - *
> - * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html
> - */
> -#define arch_counter_enforce_ordering(val) do {				\
> -	u64 tmp, _val = (val);						\
> -									\
> -	asm volatile(							\
> -	"	eor	%0, %1, %1\n"					\
> -	"	add	%0, sp, %0\n"					\
> -	"	ldr	xzr, [%0]"					\
> -	: "=r" (tmp) : "r" (_val));					\
> -} while (0)
> -
>  static __always_inline u64 __arch_counter_get_cntpct_stable(void)
>  {
>  	u64 cnt;
> @@ -224,8 +205,6 @@ static __always_inline u64 __arch_counter_get_cntvct(void)
>  	return cnt;
>  }
>  
> -#undef arch_counter_enforce_ordering
> -
>  static inline int arch_timer_arch_init(void)
>  {
>  	return 0;
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index c3009b0e5239..37d891af8ea5 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -70,6 +70,25 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx,
>  	return mask;
>  }
>  
> +/*
> + * Ensure that reads of the counter are treated the same as memory reads
> + * for the purposes of ordering by subsequent memory barriers.
> + *
> + * This insanity brought to you by speculative system register reads,
> + * out-of-order memory accesses, sequence locks and Thomas Gleixner.
> + *
> + * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html
> + */
> +#define arch_counter_enforce_ordering(val) do {				\
> +	u64 tmp, _val = (val);						\
> +									\
> +	asm volatile(							\
> +	"	eor	%0, %1, %1\n"					\
> +	"	add	%0, sp, %0\n"					\
> +	"	ldr	xzr, [%0]"					\
> +	: "=r" (tmp) : "r" (_val));					\
> +} while (0)
> +
>  #define __smp_mb()	dmb(ish)
>  #define __smp_rmb()	dmb(ishld)
>  #define __smp_wmb()	dmb(ishst)
> diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
> index 631ab1281633..4b4c0dac0e14 100644
> --- a/arch/arm64/include/asm/vdso/gettimeofday.h
> +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
> @@ -83,11 +83,7 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
>  	 */
>  	isb();
>  	asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory");
> -	/*
> -	 * This isb() is required to prevent that the seq lock is
> -	 * speculated.#
> -	 */
> -	isb();
> +	arch_counter_enforce_ordering(res);
>  
>  	return res;
>  }
> 

-- 
Regards,
Vincenzo