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

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Date:   Thu, 28 Jul 2022 22:17:31 +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 7/23/22 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.

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

Paolo