Re: [PATCH 1/5] arm64: Fix vDSO clock_getres()

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Apr 01, 2019 at 12:51:48PM +0100, Vincenzo Frascino wrote:
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 2d419006ad43..47ba72345739 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -245,6 +245,8 @@ void update_vsyscall(struct timekeeper *tk)
>  		vdso_data->cs_shift		= tk->tkr_mono.shift;
>  	}
>  
> +	vdso_data->hrtimer_res			= hrtimer_resolution;
> +
>  	smp_wmb();
>  	++vdso_data->tb_seq_count;
>  }
> diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
> index c39872a7b03c..7a2cd2f8e13a 100644
> --- a/arch/arm64/kernel/vdso/gettimeofday.S
> +++ b/arch/arm64/kernel/vdso/gettimeofday.S
> @@ -296,32 +296,35 @@ ENDPROC(__kernel_clock_gettime)
>  /* int __kernel_clock_getres(clockid_t clock_id, struct timespec *res); */
>  ENTRY(__kernel_clock_getres)
>  	.cfi_startproc
> +	adr	vdso_data, _vdso_data
>  	cmp	w0, #CLOCK_REALTIME
>  	ccmp	w0, #CLOCK_MONOTONIC, #0x4, ne
>  	ccmp	w0, #CLOCK_MONOTONIC_RAW, #0x4, ne
> -	b.ne	1f
> +	b.ne	2f
>  
> -	ldr	x2, 5f
> -	b	2f
> -1:
> +1:	/* Get hrtimer_res */
> +	seqcnt_acquire
> +	syscall_check fail=5f
> +	ldr	x2, [vdso_data, #CLOCK_REALTIME_RES]
> +	seqcnt_check fail=1b
> +	b	3f
> +2:

We talked briefly but I'm still confused why we need the fallback to the
syscall here if archdata.vdso_direct is false. Is it because if the
timer driver code sets vdso_direct to false, we don't don't support
highres timers? If my understanding is correct, you may want to move the
hrtimer_res setting in update_vsyscall() to the !use_syscall block.

-- 
Catalin