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

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Date:   Wed, 27 Jan 2021 09:14:57 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...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 Tue, Jan 26, 2021, Ben Gardon wrote:
> 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.

Yep, definitely a different series.