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

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Message-ID: <f8ccb892-cf79-62df-2752-2333467245cd@redhat.com>
Date:   Fri, 29 Jul 2022 00:09:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 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; I 
have posted a couple comments for patch 1 and 2.

One way to simplify the rmb/wmb logic could be to place the rmb/wmb 
respectively after loading iter.old_spte and in tdp_mmu_link_sp.  If you 
like it, feel free to integrate it in v3.

Paolo