[PATCH 1/2] arm64: vdso: fix coarse clock handling

[Will Deacon]

Hi Nathan,

Thanks for the patch!

On Mon, Feb 03, 2014 at 07:48:51PM +0000, Nathan Lynch wrote:
> When __kernel_clock_gettime is called with a CLOCK_MONOTONIC_COARSE or
> CLOCK_REALTIME_COARSE clock id, it returns incorrectly to whatever the
> caller has placed in x2.  Fix this by saving x30/LR to x2
> unconditionally.
> 
> Also: the tv_nsec field in the result is shifted by the value in x12.
> In the high-precision case x12 is cs_shift from the data page, but
> for coarse clocks x12 is uninitialized.  Fix this by setting x12 to 0
> once we know we are dealing with a coarse clock.

How are you managing to see these failures? It's clear that our LTP testing
isn't hitting this path...

> Signed-off-by: Nathan Lynch <nathan_lynch at mentor.com>
> ---
>  arch/arm64/kernel/vdso/gettimeofday.S | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
> index f0a6d10b5211..6c37ae4a70c0 100644
> --- a/arch/arm64/kernel/vdso/gettimeofday.S
> +++ b/arch/arm64/kernel/vdso/gettimeofday.S
> @@ -88,13 +88,13 @@ ENDPROC(__kernel_gettimeofday)
>  /* int __kernel_clock_gettime(clockid_t clock_id, struct timespec *tp); */
>  ENTRY(__kernel_clock_gettime)
>  	.cfi_startproc
> +	mov	x2, x30
> +	.cfi_register x30, x2
> +
>  	cmp	w0, #CLOCK_REALTIME
>  	ccmp	w0, #CLOCK_MONOTONIC, #0x4, ne
>  	b.ne	2f
>  
> -	mov	x2, x30
> -	.cfi_register x30, x2
> -

Could we avoid the redundant moves by instead doing this around the bl
__do_get_tspec?

>  	/* Get kernel timespec. */
>  	adr	vdso_data, _vdso_data
>  1:	seqcnt_acquire
> @@ -118,6 +118,9 @@ ENTRY(__kernel_clock_gettime)
>  	ccmp	w0, #CLOCK_MONOTONIC_COARSE, #0x4, ne
>  	b.ne	8f
>  
> +	/* Set shift to 0 for coarse clocks */
> +	mov	x12, #0

Worth mentioning that xtime_coarse_nsec is pre-shifted for us, rather than
shifting not actually being required.

Also, rather than shift by #0, can we move the lsl instruction immediately
before the b 4f earlier on?

Will