[PATCH] powerpc/vdso64: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE

[Michael Ellerman]

Santosh Sivaraj <santosh at fossix.org> writes:

> Current vDSO64 implementation does not have support for coarse
> clocks (CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME_COARSE), for which it falls
> back to system call. Below is a benchmark of the difference in execution
> time with and without vDSO support.

Hi Santosh,

Great patch! Always good to see asm replaced with C.

> diff --git a/arch/powerpc/kernel/vdso64/gettime.c b/arch/powerpc/kernel/vdso64/gettime.c
> new file mode 100644
> index 0000000..01f411f
> --- /dev/null
> +++ b/arch/powerpc/kernel/vdso64/gettime.c
> @@ -0,0 +1,162 @@
...
> +static notrace int gettime_syscall_fallback(clockid_t clk_id,
> +					     struct timespec *tp)
> +{
> +	register clockid_t id asm("r3") = clk_id;
> +	register struct timespec *t asm("r4") = tp;
> +	register int nr asm("r0") = __NR_clock_gettime;
> +	register int ret asm("r3");

I guess this works. I've always been a bit nervous about register
variables TBH.

> +	asm volatile("sc"
> +		     : "=r" (ret)
> +		     : "r"(nr), "r"(id), "r"(t)
> +		     : "memory");

Not sure we need the memory clobber?

It can clobber more registers than that though.

See: Documentation/powerpc/syscall64-abi.txt

> diff --git a/arch/powerpc/kernel/vdso64/gettimeofday.S b/arch/powerpc/kernel/vdso64/gettimeofday.S
> index 3820213..1258009 100644
> --- a/arch/powerpc/kernel/vdso64/gettimeofday.S
> +++ b/arch/powerpc/kernel/vdso64/gettimeofday.S
> @@ -51,85 +53,21 @@ V_FUNCTION_BEGIN(__kernel_gettimeofday)
...
> +	stwu	r1,-112(r1)
> +  .cfi_register lr,r6
> +	std	r6,24(r1)
> +	bl	V_LOCAL_FUNC(kernel_clock_gettime)
>  	crclr	cr0*4+so

Clearing CR0[SO] says that the syscall always succeeded.

What happens if you call this with a completely bogus clock id?

I think the solution is probably to do the syscall fallback in asm, and
everything else in C.

cheers