Re: [kvm-unit-tests PATCH v2 1/2] lib/s390x: time: add wrapper for stckf

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu,  1 Sep 2022 17:09:55 +0200
Nico Boehr <nrb@linux.ibm.com> wrote:

> Upcoming changes will do performance measurements of instructions. Since
> stck is designed to return unique values even on concurrent calls, it is
> unsuited for performance measurements. stckf should be used in this
> case.
> 
> While touching that code, also add a missing cc clobber in
> get_clock_us() and avoid the memory clobber by moving the clock value to
> the output operands.
> 
> Hence, add a nice wrapper for stckf to the time library.
> 
> Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
> ---
>  lib/s390x/asm/time.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/s390x/asm/time.h b/lib/s390x/asm/time.h
> index 7652a151e87a..8d2327a40541 100644
> --- a/lib/s390x/asm/time.h
> +++ b/lib/s390x/asm/time.h
> @@ -14,11 +14,20 @@
>  #define STCK_SHIFT_US	(63 - 51)
>  #define STCK_MAX	((1UL << 52) - 1)
>  
> +static inline uint64_t get_clock_fast(void)
> +{
> +	uint64_t clk;
> +
> +	asm volatile(" stckf %0 " : "=Q"(clk) : : "cc");
> +
> +	return clk;
> +}
> +
>  static inline uint64_t get_clock_us(void)
>  {
>  	uint64_t clk;
>  
> -	asm volatile(" stck %0 " : : "Q"(clk) : "memory");
> +	asm volatile(" stck %0 " : "=Q"(clk) : : "cc");

this fix is not needed if you use the wrapper for stck from your
other patch series

>  
>  	return clk >> STCK_SHIFT_US;
>  }