linux-kernel - Re: [PATCH 24/24] kvm: x86/mmu: Allow parallel page faults for the TDP MMU

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Date:   Tue, 26 Jan 2021 13:57:33 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        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 24/24] kvm: x86/mmu: Allow parallel page faults for the
 TDP MMU

On Wed, Jan 20, 2021 at 4:56 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Jan 12, 2021, Ben Gardon wrote:
> > Make the last few changes necessary to enable the TDP MMU to handle page
> > faults in parallel while holding the mmu_lock in read mode.
> >
> > Reviewed-by: Peter Feiner <pfeiner@...gle.com>
> >
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 280d7cd6f94b..fa111ceb67d4 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3724,7 +3724,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> >               return r;
> >
> >       r = RET_PF_RETRY;
> > -     kvm_mmu_lock(vcpu->kvm);
> > +
> > +     if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
>
> Off topic, what do you think about rewriting is_tdp_mmu_root() to be both more
> performant and self-documenting as to when is_tdp_mmu_root() !=
> kvm->arch.tdp_mmu_enabled?  E.g. key off is_guest_mode() and then do a thorough
> audit/check when CONFIG_KVM_MMU_AUDIT=y?
>
> #ifdef CONFIG_KVM_MMU_AUDIT
> bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
> {
>         struct kvm_mmu_page *sp;
>
>         if (!kvm->arch.tdp_mmu_enabled)
>                 return false;
>         if (WARN_ON(!VALID_PAGE(hpa)))
>                 return false;
>
>         sp = to_shadow_page(hpa);
>         if (WARN_ON(!sp))
>                 return false;
>
>         return sp->tdp_mmu_page && sp->root_count;
> }
> #endif
>
> bool is_tdp_mmu(struct kvm_vcpu *vcpu)
> {
>         bool is_tdp_mmu = kvm->arch.tdp_mmu_enabled && !is_guest_mode(vcpu);
>
> #ifdef CONFIG_KVM_MMU_AUDIT
>         WARN_ON(is_tdp_mmu != is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa));
> #endif
>         return is_tdp_mmu;
> }

Great suggestions. In the interest of keeping this (already enormous)
series small, I'm inclined to make those changes in a future series if
that's alright with you.

>
> > +             kvm_mmu_lock_shared(vcpu->kvm);
> > +     else
> > +             kvm_mmu_lock(vcpu->kvm);
> > +
> >       if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> >               goto out_unlock;
> >       r = make_mmu_pages_available(vcpu);
> > @@ -3739,7 +3744,10 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> >                                prefault, is_tdp);
> >
> >  out_unlock:
> > -     kvm_mmu_unlock(vcpu->kvm);
> > +     if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> > +             kvm_mmu_unlock_shared(vcpu->kvm);
> > +     else
> > +             kvm_mmu_unlock(vcpu->kvm);
> >       kvm_release_pfn_clean(pfn);
> >       return r;
> >  }
> > --
> > 2.30.0.284.gd98b1dd5eaa7-goog
> >