linux-kernel - Re: [PATCH 0/2] KVM: x86/mmu: .change

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Message-ID: <ZN92MtFkIF3E79/u@google.com>
Date:   Fri, 18 Aug 2023 06:46:26 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com
Subject: Re: [PATCH 0/2] KVM: x86/mmu: .change_pte() optimization in TDP MMU

On Fri, Aug 18, 2023, Yan Zhao wrote:
> On Thu, Aug 17, 2023 at 10:53:25AM -0700, Sean Christopherson wrote:
> > And FWIW, removing .change_pte() entirely, even without any other optimizations,
> > will also benefit those guests, as it will remove a source of mmu_lock contention
> > along with all of the overhead of invoking callbacks, walking memslots, etc.  And
> > removing .change_pte() will benefit *all* guests by eliminating unrelated callbacks,
> > i.e. callbacks when memory for the VMM takes a CoW fault.
> >
> If with above "always write_fault = true" solution, I think it's better.

Another option would be to allow a per-mm override of use_zero_page, but I think
I like the KVM memslot route more as it provides better granularity, doesn't
prevent CoW for VMM memory, and works even if THP isn't being used.