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

Paolo Bonzini
2022-07-28T22:10:04Z

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