linux-kernel - Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

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Date:   Thu, 9 Sep 2021 18:47:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Chenyi Qiang <chenyi.qiang@...el.com>, Tao Xu <tao3.xu@...el.com>,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On Tue, Sep 07, 2021, Xiaoyao Li wrote:
> On 9/3/2021 12:29 AM, Sean Christopherson wrote:
> > > After syncing internally, we know that the internal threshold is not
> > > architectural but a model-specific value. It will be published in some place
> > > in future.
> > 
> > Any chance it will also be discoverable, e.g. via an MSR?
> 
> I also hope we can expose it via MSR. If not, we can maintain a table per
> FMS in KVM to get the internal threshold. However, per FMS info is not
> friendly to be virtualized (when we are going to enable the nested support).

Yeah, FMS is awful.  If the built-in buffer isn't discoverable, my vote is to
assume the worst, i.e. a built-in buffer of '0', and have the notify_window
param default to a safe value, e.g. 25k or maybe even 150k (to go above what the
hardware folks apparently deemed safe for SPR).  It's obviously not idea, but
it's better than playing FMS guessing games.

> I'll try to persuade internal to expose it via MSR, but I guarantee nothing.

...

> > On a related topic, this needs tests.  One thought would be to stop unconditionally
> > intercepting #AC if NOTIFY_WINDOW is enabled, and then have the test set up the
> > infinite #AC vectoring scenario.
> > 
> 
> yes, we have already tested with this case with notify_window set to 0. No
> false positive.

Can you send a selftest or kvm-unit-test?