linux-kernel - Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in exclusive mode

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Message-ID: <YEk2kBRUriFlCM62@google.com>
Date:   Wed, 10 Mar 2021 13:13:52 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP
 in exclusive mode

On Wed, Mar 10, 2021, Paolo Bonzini wrote:
> On 10/03/21 01:30, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 50ef757c5586..f0c99fa04ef2 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -323,7 +323,18 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
> >   				cpu_relax();
> >   			}
> >   		} else {
> > +			/*
> > +			 * If the SPTE is not MMU-present, there is no backing
> > +			 * page associated with the SPTE and so no side effects
> > +			 * that need to be recorded, and exclusive ownership of
> > +			 * mmu_lock ensures the SPTE can't be made present.
> > +			 * Note, zapping MMIO SPTEs is also unnecessary as they
> > +			 * are guarded by the memslots generation, not by being
> > +			 * unreachable.
> > +			 */
> >   			old_child_spte = READ_ONCE(*sptep);
> > +			if (!is_shadow_present_pte(old_child_spte))
> > +				continue;
> >   			/*
> >   			 * Marking the SPTE as a removed SPTE is not
> 
> Ben, do you plan to make this path take mmu_lock for read?  If so, this
> wouldn't be too useful IIUC.

I can see kvm_mmu_zap_all_fast()->kvm_tdp_mmu_zap_all() moving to a shared-mode
flow, but I don't think we'll ever want to move away from exclusive-mode zapping
for kvm_arch_flush_shadow_all()->kvm_mmu_zap_all()->kvm_tdp_mmu_zap_all().  In
that case, the VM is dead or dying; freeing memory should be done as quickly as
possible.