linux-kernel - Re: [PATCH] selftests/kvm: add test for KVM_GET_MSR_FEATURE_INDEX

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Message-ID: <20210317120714.6xuqujpvdn2tzddj@kamzik.brq.redhat.com>
Date:   Wed, 17 Mar 2021 13:07:14 +0100
From:   Andrew Jones <drjones@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        linux-kselftest@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] selftests/kvm: add test for
 KVM_GET_MSR_FEATURE_INDEX_LIST

On Wed, Mar 17, 2021 at 12:25:52PM +0100, Emanuele Giuseppe Esposito wrote:
> 
> 
> On 17/03/2021 11:49, Paolo Bonzini wrote:
> > On 17/03/21 08:45, Emanuele Giuseppe Esposito wrote:
> > > +    struct kvm_msr_list features_list;
> > >       buffer.header.nmsrs = 1;
> > >       buffer.entry.index = msr_index;
> > > +    features_list.nmsrs = 1;
> > > +
> > >       kvm_fd = open(KVM_DEV_PATH, O_RDONLY);
> > >       if (kvm_fd < 0)
> > >           exit(KSFT_SKIP);
> > > +    r = ioctl(kvm_fd, KVM_GET_MSR_FEATURE_INDEX_LIST, &features_list);
> > > +    TEST_ASSERT(r < 0 && r != -E2BIG,
> > > "KVM_GET_MSR_FEATURE_INDEX_LIST IOCTL failed,\n"
> > > +        "  rc: %i errno: %i", r, errno);
> > 
> > Careful: because this has nsmrs == 1, you are overwriting an u32 of the
> > stack after struct kvm_msr_list.  You need to use your own struct
> > similar to what is done with "buffer.header" and "buffer.entry".
> > 
> > >       r = ioctl(kvm_fd, KVM_GET_MSRS, &buffer.header);
> > >       TEST_ASSERT(r == 1, "KVM_GET_MSRS IOCTL failed,\n"
> > >           "  rc: %i errno: %i", r, errno);
> > > 
> > 
> > More in general, this is not a test, but rather a library function used
> > to read a single MSR.
> > 
> > If you would like to add a test for KVM_GET_MSR_FEATURE_INDEX_LIST that
> > would be very welcome.  That would be a new executable.  Looking at the
> > logic for the ioctl, the main purpose of the test should be:
> > 
> > - check that if features_list.nmsrs is too small it will set the nmsrs
> > field and return -E2BIG.
> > 
> > - check that all MSRs returned by KVM_GET_MSR_FEATURE_INDEX_LIST can be
> > accessed with KVM_GET_MSRS
> > 
> > So something like this:
> > 
> >    set nmsrs to 0 and try the ioctl
> >    check that it returns -E2BIG and has changed nmsrs
> >    if nmsrs != 1 {
> >      set nmsrs to 1 and try the ioctl again
> >      check that it returns -E2BIG
> >    }
> >    malloc a buffer with room for struct kvm_msr_list and nmsrs indices
> >    set nmsrs in the malloc-ed buffer and try the ioctl again
> >    for each index
> >      invoke kvm_get_feature_msr to read it
> > 
> > (The test should also be skipped if KVM does not expose the
> > KVM_CAP_GET_MSR_FEATURES capability).
> 
> Thank you for the feedback, the title is indeed a little bit misleading. My
> idea in this patch was to just add an additional check to all usages of
> KVM_GET_MSRS, since KVM_GET_MSR_FEATURE_INDEX_LIST is used only to probe
> host capabilities and processor features.
> But you are right, a separate test would be better.
>

Hi Emanuele,

You might be able to get some inspiration from the aarch64/get-reg-list.c
test. The list of MSRs varies with KVM version and host processor, but
there may be a set of MSRs that does not vary with host processor and
should not be removed in later KVM versions. If that's the case, then
the !missing_regs assert concept of aarch64/get-reg-list.c may also
apply to this new test. Based on Paolo's comment, I presume at least the
!failed_get should apply. Finally, the test should do the E2BIG checks,
as Paolo states, but you may also want to create a lib function for
KVM_GET_MSR_FEATURE_INDEX_LIST, similar to vcpu_get_reg_list(), if you
think it may be of use to other tests.

Thanks,
drew