Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> On Thu, Sep 22, 2022, Vitaly Kuznetsov wrote:
>> diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_features.c b/tools/testing/selftests/kvm/x86_64/hyperv_features.c
>> index d4bd18bc580d..18b44450dfb8 100644
>> --- a/tools/testing/selftests/kvm/x86_64/hyperv_features.c
>> +++ b/tools/testing/selftests/kvm/x86_64/hyperv_features.c
>> @@ -46,20 +46,33 @@ struct hcall_data {
>>
>> static void guest_msr(struct msr_data *msr)
>> {
>> - uint64_t ignored;
>> + uint64_t msr_val = 0;
>> uint8_t vector;
>>
>> GUEST_ASSERT(msr->idx);
>>
>> - if (!msr->write)
>> - vector = rdmsr_safe(msr->idx, &ignored);
>> - else
>> + if (!msr->write) {
>> + vector = rdmsr_safe(msr->idx, &msr_val);
>
> This is subtly going to do weird things if the RDMSR faults. rdmsr_safe()
> overwrites @val with whatever happens to be in EDX:EAX if the RDMSR faults, i.e.
> this may yield garbage instead of '0'. Arguably rdmsr_safe() is a bad API, but
> at the same time the caller really shouldn't consume the result if RDMSR faults
> (though aligning with the kernel is also valuable).
>
> Aha! Idea. Assuming none of the MSRs are write-only, what about adding a prep
> patch to rework this code so that it verifies RDMSR returns what was written when
> a fault didn't occur.
>
There is at least one read-only MSR which comes to mind:
HV_X64_MSR_EOI. Also, some of the MSRs don't preserve the written value,
e.g. HV_X64_MSR_RESET which always reads as '0'.
I do, however, like the additional check that RDMSR returns what was
written to the MSR, we will just need an additional flag in 'struct
msr_data' ('check_written_value' maybe?).
--
Vitaly
