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

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Message-ID: <61d11f32-a2da-b593-1c62-bbadc6408215@redhat.com>
Date:   Wed, 17 Mar 2021 12:25:52 +0100
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        linux-kselftest@...r.kernel.org
Cc:     Shuah Khan <shuah@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Andrew Jones <drjones@...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 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.

Thank you,
Emanuele