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

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 28, 2022, Paolo Bonzini wrote:
> On 7/23/22 03:23, Sean Christopherson wrote:
> > Patch 6 from Mingwei is the end goal of the series.  KVM incorrectly
> > assumes that the NX huge page mitigation is the only scenario where KVM
> > will create a non-leaf page instead of a huge page.   Precisely track
> > (via kvm_mmu_page) if a non-huge page is being forced and use that info
> > to avoid unnecessarily forcing smaller page sizes in
> > disallowed_hugepage_adjust().
> > 
> > v2: Rebase, tweak a changelog accordingly.
> > 
> > v1:https://lore.kernel.org/all/20220409003847.819686-1-seanjc@google.com
> > 
> > Mingwei Zhang (1):
> >    KVM: x86/mmu: explicitly check nx_hugepage in
> >      disallowed_hugepage_adjust()
> > 
> > Sean Christopherson (5):
> >    KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked
> >    KVM: x86/mmu: Properly account NX huge page workaround for nonpaging
> >      MMUs
> >    KVM: x86/mmu: Set disallowed_nx_huge_page in TDP MMU before setting
> >      SPTE
> >    KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual
> >      pages
> >    KVM: x86/mmu: Add helper to convert SPTE value to its shadow page
> 
> Some of the benefits are cool, such as not having to track the pages for the
> TDP MMU, and patch 2 is a borderline bugfix, but there's quite a lot of new
> non-obvious complexity here.

100% agree on the complexity.

> So the obligatory question is: is it worth a hundred lines of new code?

Assuming I understanding the bug Mingwei's patch fixes, yes.  Though after
re-reading that changelog, it should more explicitly call out the scenario we
actually care about.

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.