linux-kernel - Re: [PATCH v5 09/26] KVM: VMX: nVMX: Support TSC scaling and PERF_GLOBAL

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Message-ID: <875yikxj6w.fsf@redhat.com>
Date:   Mon, 22 Aug 2022 19:49:11 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 09/26] KVM: VMX: nVMX: Support TSC scaling and
 PERF_GLOBAL_CTRL with enlightened VMCS

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

> On Mon, Aug 22, 2022, Vitaly Kuznetsov wrote:
>> Sean Christopherson <seanjc@...gle.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