linux-kernel - Re: [PATCH v2 0/6] KVM: x86: Apply NX mitigation more precisely

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Date:   Thu, 28 Jul 2022 22:15:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 0/6] KVM: x86: Apply NX mitigation more precisely

On Fri, Jul 29, 2022, Paolo Bonzini wrote:
> On 7/28/22 23:20, Sean Christopherson wrote:
> > 
> > Anyways, the bug we really care about is that by not precisely checking if a
> > huge page is disallowed, KVM would refuse to create huge page after disabling
> > dirty logging, which is a very noticeable performance issue for large VMs if
> > a migration is canceled.  That particular bug has since been unintentionally
> > fixed in the TDP MMU by zapping the non-leaf SPTE, but there are other paths
> > that could similarly be affected, e.g. I believe zapping leaf SPTEs in response
> > to a host page migration (mmu_notifier invalidation) to create a huge page would
> > yield a similar result; KVM would see the shadow-present non-leaf SPTE and assume
> > a huge page is disallowed.
> 
> Ok, thanks.  So this will be 5.21 material even during the -rc phase

Yes, this definitely needs more time in the queue before being sent to Linus.