linux-kernel - Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs

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Date:   Thu, 12 Aug 2021 17:07:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator
 when zapping all SPTEs

On Thu, Aug 12, 2021, Ben Gardon wrote:
> On Wed, Aug 11, 2021 at 10:07 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Set the min_level for the TDP iterator at the root level when zapping all
> > SPTEs so that the _iterator_ only processes top-level SPTEs.  Zapping a
> > non-leaf SPTE will recursively zap all its children, thus there is no
> > need for the iterator to attempt to step down.  This avoids rereading all
> > the top-level SPTEs after they are zapped by causing try_step_down() to
> > short-circuit.
> >
> > Cc: Ben Gardon <bgardon@...gle.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 
> This change looks functionally correct, but I'm not sure it's worth
> adding more code special cased on zap-all for what seems like a small
> performance improvement in a context which shouldn't be particularly
> performance sensitive.

Yeah, I was/am on the fence too, I almost included a blurb in the cover letter
saying as much.  I'll do that for v2 and let Paolo decide.

Thanks!

> Change is a correct optimization though and it's not much extra code,
> so I'm happy to give a:
> Reviewed-by: Ben Gardon <bgardon@...gle.com>