linux-kernel - Re: [PATCH v2 20/27] KVM: x86/mmu: Use page-track notifiers iff there are external users

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Date:   Wed, 15 Mar 2023 09:54:30 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 20/27] KVM: x86/mmu: Use page-track notifiers iff there
 are external users

On Wed, Mar 15, 2023, Yan Zhao wrote:
> On Fri, Mar 10, 2023 at 04:22:51PM -0800, Sean Christopherson wrote:
> > Disable the page-track notifier code at compile time if there are no
> > external users, i.e. if CONFIG_KVM_EXTERNAL_WRITE_TRACKING=n.  KVM itself
> > now hooks emulated writes directly instead of relying on the page-track
> > mechanism.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h       |  2 ++
> >  arch/x86/include/asm/kvm_page_track.h |  2 ++
> >  arch/x86/kvm/mmu/page_track.c         |  9 ++++-----
> >  arch/x86/kvm/mmu/page_track.h         | 29 +++++++++++++++++++++++----
> >  4 files changed, 33 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 1a4225237564..a3423711e403 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1265,7 +1265,9 @@ struct kvm_arch {
> >  	 * create an NX huge page (without hanging the guest).
> >  	 */
> >  	struct list_head possible_nx_huge_pages;
> > +#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING
> >  	struct kvm_page_track_notifier_head track_notifier_head;
> > +#endif
> >  	/*
> >  	 * Protects marking pages unsync during page faults, as TDP MMU page
> >  	 * faults only take mmu_lock for read.  For simplicity, the unsync
> > diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> > index deece45936a5..53c2adb25a07 100644
> > --- a/arch/x86/include/asm/kvm_page_track.h
> > +++ b/arch/x86/include/asm/kvm_page_track.h
> The "#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING" can be moved to the
> front of this file?
> All the structures are only exposed for external users now.

Huh.  I've no idea why I didn't do that.  IIRC, the entire reason past me wrapped
track_notifier_head in an #ifdef was to allow this change in kvm_page_track.h.

I'll do this in the next version unless I discover an edge case I'm overlooking.

Thanks yet again!