linux-kernel - Re: [PATCH 0/2] KVM: x86/mmu: Use page-track only for... page tracking

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Date:   Fri, 11 Nov 2022 19:24:55 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 0/2] KVM: x86/mmu: Use page-track only for... page
 tracking

On Thu, Nov 10, 2022, Sean Christopherson wrote:
> Don't bounce through the page-track notifier when zapping+flushing SPTEs
> in response to memslot changes as the need to zap+flush isn't strictly
> limited to page-tracking.  With that done, register KVM's notifier on the
> first allocation of a shadow root, as KVM's ->track_write() hook is used
> only to react to writes to gPTEs.
> 
> Aside from avoiding a RETPOLINE on emulated writes, dropping KVM's internal
> use will allow removing ->track_flush_slot() altogether once KVM-GT moves
> to a different hook[*].  Tracking "flushes" of slots is a poor fit for
> KVM-GT's needs as KVM-GT needs to drop its write-protection only when a
> memslot change is guaranteed to be committed, whereas the "flush" call is
> speculative in the sense that KVM may abort a memslot update after flushing
> the original memslot.
> 
> https://lore.kernel.org/all/20221108084416.11447-1-yan.y.zhao@intel.com
> 
> Sean Christopherson (2):
>   KVM: x86/mmu: Don't rely on page-track mechanism to flush on memslot
>     change
>   KVM: x86/mmu: Register page-tracker on first shadow root allocation

Don't merge this series, I'm going to (hopefully) send a (much larger) v2 that
more aggressively cleans up the page tracker APIs, and will replace patch 2 with
a completely different patch.