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

[Yan Zhao]

On Tue, Jun 25, 2024 at 08:51:33AM +0800, Edgecombe, Rick P wrote:
> 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?
IMO it's a concrete problem, though rare up to now. e.g.

After repeatedly hot-plugping and hot-unplugping memory, which increases
memslots generation, kvm_mmu_zap_all_fast() will be called to invalidate direct
roots when the memslots generation wraps around.

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