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

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Date:   Fri, 25 Feb 2022 20:25:26 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.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 8:07 PM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
>
> On 2/25/2022 11:13 PM, Paolo Bonzini 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.
>
> IMO, a well written VMM (in L1) should handle it correctly.
>
> L0 KVM reports no Notify VM Exit support to L1, so L1 runs without
> setting Notify VM exit. If a L2 causes notify_vm_exit with
> invalid_vm_context, L0 just reflects it to L1. In L1's view, there is no
> support of Notify VM Exit from VMX MSR capability. Following L1 handler
> is possible:
>
> a)      if (notify_vm_exit available & notify_vm_exit enabled) {
>                 handle in b)
>         } else {
>                 report unexpected vm exit reason to userspace;
>         }
>
> b)      similar handler like we implement in KVM:
>         if (!vm_context_invalid)
>                 re-enter guest;
>         else
>                 report to userspace;
>
> c)      no Notify VM Exit related code (e.g. old KVM), it's treated as
> unsupported exit reason
>
> As long as it belongs to any case above, I think L1 can handle it
> correctly. Any nasty occurrence should be caused by incorrect handler in
> L1 VMM, in my opinion.

Please test some common hypervisors (e.g. ESXi and Hyper-V).