linux-kernel - Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

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Message-ID: <Yhkf/qJ1wpfbA3Fc@google.com>
Date:   Fri, 25 Feb 2022 18:29:18 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

On Fri, Feb 25, 2022, Jim Mattson wrote:
> On Fri, Feb 25, 2022 at 7:13 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On 2/25/22 16:12, Xiaoyao Li wrote:
> > >>>>
> > >>>
> > >>> I don't like the idea of making things up without notifying userspace
> > >>> that this is fictional. How is my customer running nested VMs supposed
> > >>> to know that L2 didn't actually shutdown, but L0 killed it because the
> > >>> notify window was exceeded? If this information isn't reported to
> > >>> userspace, I have no way of getting the information to the customer.
> > >>
> > >> Then, maybe a dedicated software define VM exit for it instead of
> > >> reusing triple fault?
> > >>
> > >
> > > Second thought, we can even just return Notify VM exit to L1 to tell L2
> > > causes Notify VM exit, even thought Notify VM exit is not exposed to L1.
> >
> > That might cause NULL pointer dereferences or other nasty occurrences.
> 
> Could we synthesize a machine check? I haven't looked in detail at the
> MCE MSRs, but surely there must be room in there for Intel to reserve
> some encodings for synthesized machine checks.

I don't think we have any choice but to synthesize SHUTDOWN until we get more
details on the exact semantics of VM_CONTEXT_INVALID.  E.g. if GUEST_EFER or any
other critical guest field is corrupted, attempting to re-enter the guest, even
to (attempt to) inject a machine check, is risking undefined behavior in the guest.