linux-kernel - Re: [PATCH v4 6/6] KVM: selftests: Test Hyper-V invariant TSC control

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Message-ID: <87pmew6q3d.fsf@ovpn-194-196.brq.redhat.com>
Date:   Thu, 13 Oct 2022 11:16:22 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/6] KVM: selftests: Test Hyper-V invariant TSC control

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Oct 12, 2022, Vitaly Kuznetsov wrote:
>> Sean Christopherson <seanjc@...gle.com> writes:
>> 

...

>> > 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.
>
> I assume s/read-only/write-only since it's EOI?
>

Yes, of course)

>> Also, some of the MSRs don't preserve the written value,
>> e.g. HV_X64_MSR_RESET which always reads as '0'.
>
> Hrm, that's annoying.

'Slightly annoying'. In fact, the test never writes anything besides '0'
to the MSR as the code is not ready to handle real vCPU reset. I'll
leave a TODO note about that.

...

> static bool is_write_only_msr(uint32_t msr)
> {
> 	return msr == HV_X64_MSR_EOI;
> }

This is all we need, basically. I'll go with that.

-- 
Vitaly