Re: [kvm-unit-tests PATCH v1 1/2] arm/pmu: skip the PMU introspection test if missing

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

Hi,

On Tue, Jul 02, 2024 at 05:35:14PM +0100, Alex Bennée wrote:
> The test for number of events is not a substitute for properly
> checking the feature register. Fix the define and skip if PMUv3 is not
> available on the system. This includes emulator such as QEMU which
> don't implement PMU counters as a matter of policy.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Anders Roxell <anders.roxell@linaro.org>
> ---
>  arm/pmu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arm/pmu.c b/arm/pmu.c
> index 9ff7a301..66163a40 100644
> --- a/arm/pmu.c
> +++ b/arm/pmu.c
> @@ -200,7 +200,7 @@ static void test_overflow_interrupt(bool overflow_at_64bits) {}
>  #define ID_AA64DFR0_PERFMON_MASK  0xf
>  
>  #define ID_DFR0_PMU_NOTIMPL	0b0000
> -#define ID_DFR0_PMU_V3		0b0001
> +#define ID_DFR0_PMU_V3		0b0011
>  #define ID_DFR0_PMU_V3_8_1	0b0100
>  #define ID_DFR0_PMU_V3_8_4	0b0101
>  #define ID_DFR0_PMU_V3_8_5	0b0110
> @@ -286,6 +286,11 @@ static void test_event_introspection(void)
>  		return;
>  	}
>  
> +	if (pmu.version < ID_DFR0_PMU_V3) {
> +		report_skip("PMUv3 extensions not supported, skip ...");
> +		return;
> +	}
> +

I don't get this patch - test_event_introspection() is only run on 64bit. On
arm64, if there is a PMU present, that PMU is a PMUv3.  A prerequisite to
running any PMU tests is for pmu_probe() to succeed, and pmu_probe() fails if
there is no PMU implemented (PMUVer is either 0, or 0b1111). As a result, if
test_event_introspection() is executed, then a PMUv3 is present.

When does QEMU advertise FEAT_PMUv3*, but no event counters (other than the cycle
counter)?

If you want to be extra correct, you can add the above check to pmu_probe() for
32bit, since I doubt that the PMU tests were designed or tested on anything
other than a PMUv3 (and probably not much interest to maintain the tests for
PMUv1 or v2 either). If do do this, may I suggest:

#if defined(__arm__)
	if (pmu.version == ID_DFR0_PMU_V1 || pmu.version == ID_DFR0_PMU_V2)
		return false;
#endif

That way the check is self documenting.

Thanks,
Alex

>  	/* PMUv3 requires an implementation includes some common events */
>  	required_events = is_event_supported(SW_INCR, true) &&
>  			  is_event_supported(CPU_CYCLES, true) &&
> -- 
> 2.39.2
>