Re: [kvm-unit-tests PATCH 1/1] arm64: microbench: Move the read of the count register and the ISB operation out of the while loop

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Tue, Nov 07, 2023 at 05:51:15PM +0800, heqiong wrote:
> Reducing the impact of the cntvct_el0 register and isb() operation
> on microbenchmark test results to improve testing accuracy and reduce
> latency in test results.
> ---
>  arm/micro-bench.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arm/micro-bench.c b/arm/micro-bench.c
> index fbe59d03..65f4c4dd 100644
> --- a/arm/micro-bench.c
> +++ b/arm/micro-bench.c
> @@ -346,17 +346,21 @@ static void loop_test(struct exit_test *test)
>  		}
>  	}
>  
> +	dsb(ish);
> +	isb();
> +	start = read_sysreg(cntvct_el0);
> +	isb();
>  	while (ntimes < test->times && total_ns.ns < NS_5_SECONDS) {
				       ^^^^^^^^^^^^^^^^^^^^^^^^^^
This will always evaluate to true because total_ns is now computed at the
end of the loop instead of every iteration.

Do we want to drop the upper bound on how long a test takes to execute? I
don't have an opinion about it.

Thanks,
Alex

> -		isb();
> -		start = read_sysreg(cntvct_el0);
>  		test->exec();
> -		isb();
> -		end = read_sysreg(cntvct_el0);
>  
>  		ntimes++;
> -		total_ticks += (end - start);
> -		ticks_to_ns_time(total_ticks, &total_ns);
>  	}
> +	dsb(ish);
> +	isb();
> +	end = read_sysreg(cntvct_el0);
> +
> +	total_ticks = end - start;
> +	ticks_to_ns_time(total_ticks, &total_ns);
>  
>  	if (test->post) {
>  		test->post(ntimes, &total_ticks);
> -- 
> 2.39.3
>