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

[Paolo Bonzini]

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@xxxxxxxxxx
>
> 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.

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

Paolo