linux-kernel - Re: [PATCH 22/22] kvm: mmu: Don't clear write flooding count for direct roots

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Message-ID: <CANgfPd_9cvCQCNkqMZiBq0FjxYDVpXmEERKEBujREZrOOmt04w@mail.gmail.com>
Date:   Tue, 6 Oct 2020 09:19:02 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.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 22/22] kvm: mmu: Don't clear write flooding count for
 direct roots

On Mon, Oct 5, 2020 at 5:07 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Mon, Oct 05, 2020 at 03:48:09PM -0700, Ben Gardon wrote:
> > On Fri, Sep 25, 2020 at 6:25 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> > >
> > > On 25/09/20 23:23, Ben Gardon wrote:
> > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > index 42dde27decd75..c07831b0c73e1 100644
> > > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > @@ -124,6 +124,18 @@ static struct kvm_mmu_page *find_tdp_mmu_root_with_role(
> > > >       return NULL;
> > > >  }
> > > >
> > > > +hpa_t kvm_tdp_mmu_root_hpa_for_role(struct kvm *kvm,
> > > > +                                 union kvm_mmu_page_role role)
> > > > +{
> > > > +     struct kvm_mmu_page *root;
> > > > +
> > > > +     root = find_tdp_mmu_root_with_role(kvm, role);
> > > > +     if (root)
> > > > +             return __pa(root->spt);
> > > > +
> > > > +     return INVALID_PAGE;
> > > > +}
> > > > +
> > > >  static union kvm_mmu_page_role page_role_for_level(struct kvm_vcpu *vcpu,
> > > >                                                  int level)
> > > >  {
> > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > > > index cc0b7241975aa..2395ffa71bb05 100644
> > > > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > > > @@ -9,6 +9,8 @@ void kvm_mmu_init_tdp_mmu(struct kvm *kvm);
> > > >  void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm);
> > > >
> > > >  bool is_tdp_mmu_root(struct kvm *kvm, hpa_t root);
> > > > +hpa_t kvm_tdp_mmu_root_hpa_for_role(struct kvm *kvm,
> > > > +                                 union kvm_mmu_page_role role);
> > > >  hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
> > > >  void kvm_tdp_mmu_put_root_hpa(struct kvm *kvm, hpa_t root_hpa);
> > > >
> > >
> > > Probably missing a piece since this code is not used and neither is the
> > > new argument to is_root_usable.
> > >
> > > I'm a bit confused by is_root_usable since there should be only one PGD
> > > for the TDP MMU (the one for the root_mmu).
> >
> > *facepalm* sorry about that. This commit used to be titled "Implement
> > fast CR3 switching for the TDP MMU" but several refactors later most
> > of it was not useful. The only change that should be part of this
> > patch is the one to avoid clearing the write flooding counts. I must
> > have failed to revert the other changes.
>
> Tangentially related, isn't it possible to end up with multiple roots if the
> MAXPHYSADDR is different between vCPUs?  I.e. if userspace coerces KVM into
> using a mix of 4-level and 5-level EPT?
>
> Not saying that's a remotely valid config...

We'll also end up with multiple TDP MMU roots if using SMM, and being
able to switch back and forth between "legacy/shadow MMU" roots and
TDP MMU roots improves nested performance since we can use the TDP MMU
for L1.
Since the TDP MMU associates struct kvm_mmu_pages with all its roots,
no special casing should be needed for root switching.
At one point in this patch set I was using some alternative data
structure to replace struct kvm_mmu_page for the TDP MMU, but I
abandoned that approach.