linux-kernel - Re: [PATCH 13/22] kvm: mmu: Support invalidate range MMU notifier for TDP MMU

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Message-ID: <CANgfPd8hgo+HFNVrpce3Nq8gL54=frL7cjf7KC6v=ORz_zGtKw@mail.gmail.com>
Date:   Wed, 30 Sep 2020 16:27:55 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 13/22] kvm: mmu: Support invalidate range MMU notifier for
 TDP MMU

On Wed, Sep 30, 2020 at 4:24 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Wed, Sep 30, 2020 at 04:15:17PM -0700, Ben Gardon wrote:
> > On Wed, Sep 30, 2020 at 10:04 AM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > index 52d661a758585..0ddfdab942554 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -1884,7 +1884,14 @@ static int kvm_handle_hva(struct kvm *kvm, unsigned long hva,
> > > >  int kvm_unmap_hva_range(struct kvm *kvm, unsigned long start, unsigned long end,
> > > >                       unsigned flags)
> > > >  {
> > > > -     return kvm_handle_hva_range(kvm, start, end, 0, kvm_unmap_rmapp);
> > > > +     int r;
> > > > +
> > > > +     r = kvm_handle_hva_range(kvm, start, end, 0, kvm_unmap_rmapp);
> > > > +
> > > > +     if (kvm->arch.tdp_mmu_enabled)
> > > > +             r |= kvm_tdp_mmu_zap_hva_range(kvm, start, end);
> > >
> > > Similar to an earlier question, is this intentionally additive, or can this
> > > instead by:
> > >
> > >         if (kvm->arch.tdp_mmu_enabled)
> > >                 r = kvm_tdp_mmu_zap_hva_range(kvm, start, end);
> > >         else
> > >                 r = kvm_handle_hva_range(kvm, start, end, 0, kvm_unmap_rmapp);
> > >
> >
> > It is intentionally additive so the legacy/shadow MMU can handle nested.
>
> Duh.  Now everything makes sense.  I completely spaced on nested EPT.
>
> I wonder if would be worth adding a per-VM sticky bit that is set when an
> rmap is added so that all of these flows can skip the rmap walks when using
> the TDP MMU without a nested guest.

We actually do that in the full version of this whole TDP MMU scheme.
It works very well.
I'm not sure why I didn't include that in this patch set - probably
just complexity. I'll definitely include that as an optimization along
with the lazy rmap allocation in the followup patch set.