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

[Edgecombe, Rick P]

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;
> +       }