linux-kernel - Re: [PATCH v3 13/17] KVM: x86/tdp_mmu: Support mirror root for TDP MMU

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Date: Tue, 25 Jun 2024 00:51:33 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v3 13/17] KVM: x86/tdp_mmu: Support mirror root for TDP
 MMU

On Mon, 2024-06-24 at 16:30 +0800, Yan Zhao wrote:
> On Wed, Jun 19, 2024 at 03:36:10PM -0700, Rick Edgecombe wrote:
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e9c1783a8743..287dcc2685e4 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3701,7 +3701,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu
> > *vcpu)
> >         int r;
> >   
> >         if (tdp_mmu_enabled) {
> > -               kvm_tdp_mmu_alloc_root(vcpu);
> > +               if (kvm_has_mirrored_tdp(vcpu->kvm))
> > +                       kvm_tdp_mmu_alloc_root(vcpu, true);
> > +               kvm_tdp_mmu_alloc_root(vcpu, false);
> >                 return 0;
> >         }
> mmu_alloc_direct_roots() is called when vcpu->arch.mmu->root.hpa is
> INVALID_PAGE
> in kvm_mmu_reload(), which could happen after direct root is invalidated.
> 
> > -void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
> > +void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu, bool mirror)
> >   {
> >         struct kvm_mmu *mmu = vcpu->arch.mmu;
> >         union kvm_mmu_page_role role = mmu->root_role;
> > @@ -241,6 +246,9 @@ void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
> >         struct kvm *kvm = vcpu->kvm;
> >         struct kvm_mmu_page *root;
> >   
> > +       if (mirror)
> > +               role.is_mirror = 1;
> > +
> Could we add a validity check of mirror_root_hpa to prevent an incorrect ref
> count increment of the mirror root?

I was originally suspicious of the asymmetry of the tear down of mirror and
direct roots vs the allocation. Do you see a concrete problem, or just
advocating for safety?

> 
> +       if (mirror) {
> +               if (mmu->mirror_root_hpa != INVALID_PAGE)
> +                       return;
> +
>                 role.is_mirror = true;
> +       }