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SubjectRe: [PATCH v5 09/26] KVM: VMX: nVMX: Support TSC scaling and PERF_GLOBAL_CTRL with enlightened VMCS
Date
Sean Christopherson <seanjc@google.com> writes:

> On Mon, Aug 22, 2022, Vitaly Kuznetsov wrote:
>> Sean Christopherson <seanjc@google.com> writes:
>> > But that also raises the question of whether or not KVM should honor hyperv_enabled
>> > when filtering MSRs. Same question for nested VM-Enter. nested_enlightened_vmentry()
>> > will "fail" without an assist page, and the guest can't set the assist page without
>> > hyperv_enabled==true, but nothing prevents the host from stuffing the assist page.
>>
>> The case sounds more like a misbehaving VMM to me. It would probably be
>> better to fail nested_enlightened_vmentry() immediately on !hyperv_enabled.
>
> Hmm, sort of. If KVM fails explicitly fails nested VM-Enter, then allowing the
> guest to read the VMX MSRs with the same buggy setup is odd, e.g. nested VMX is
> effectively unsupported at that point since there is nothing the guest can do to
> make nested VM-Enter succeed. Extending the "fail VM-Enter" behavior would be to
> inject #GP on RDMSR, and at that point KVM is well into "made up architecture"
> behavior.
>
> All in all, I don't think it's worth forcing the issue, even though I do agree that
> the VMM is being weird if it's enabling KVM_CAP_HYPERV_ENLIGHTENED_VMCS but not
> advertising Hyper-V.

I keep thinking about KVM-on-KVM using Hyper-V features like eVMCS, eMSR
bitmap, 'l2' tlb flush,... when I can't sleep at night sometimes :-)

...

>>
>> Thanks for the thorough review here and don't hesitate to speak up when
>> you think it's too much of a change to do upon queueing)
>
> Heh, this definitely snowballed beyond "fixup on queue". Let's sort out how to
> address the filtering issue and then decide how to handle v6.
>

Yep, let's keep the snowball rolling! :-)

--
Vitaly

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