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

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Date:   Fri, 25 Feb 2022 10:06:48 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Sean Christopherson <seanjc@...gle.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 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.