Re: [PATCH v5 3/3] qtests/arm: add some mte tests

[Cornelia Huck]

On Mon, Feb 06 2023, Eric Auger <eauger@redhat.com> wrote:

> Hi,
>
> On 2/3/23 14:44, Cornelia Huck wrote:
>> @@ -517,6 +583,13 @@ static void test_query_cpu_model_expansion_kvm(const 
>> void *data)
>>          assert_set_feature(qts, "host", "pmu", false);
>>          assert_set_feature(qts, "host", "pmu", true);
>>  
>> +        /*
>> +         * Unfortunately, there's no easy way to test whether this instance
>> +         * of KVM supports MTE. So we can only assert that the feature
>> +         * is present, but not whether it can be toggled.
>> +         */
>> +        assert_has_feature(qts, "host", "mte");
> I know you replied in v4 but I am still confused:
> What does
>       (QEMU) query-cpu-model-expansion type=full model={"name":"host"}
> return on a MTE capable host and and on a non MTE capable host?

FWIW, it's "auto" in both cases, but the main problem is actually
something else...

>
> If I remember correctly qmp_query_cpu_model_expansion loops over the
> advertised features and try to set them explicitly so if the host does
> not support it this should fail and the result should be different from
> the case where the host supports it (even if it is off by default)
>
> Does assert_has_feature_enabled() returns false?

I poked around a bit with qmp on a system (well, model) with MTE where
starting a guest with MTE works just fine. I used the minimal setup
described in docs/devel/writing-monitor-commands.rst, and trying to do a
cpu model expansion with mte=on fails because the KVM ioctl fails with
-EINVAL (as we haven't set up proper memory mappings). The qtest setup
doesn't do any proper setup either AFAICS, so enabling MTE won't work
even if KVM and the host support it. (Trying to enable MTE on a host
that doesn't support it would also report an error, but a different one,
as KVM would not support the MTE cap at all.) We don't really know
beforehand what to expect ("auto" is not yet expanded, see above), so
I'm not sure how to test this in a meaningful way, even if we did set up
memory mappings (which seems like overkill for a feature test.)

The comment describing this could be improved, though :)