On Mon, Oct 5, 2020 at 5:07 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Oct 05, 2020 at 03:48:09PM -0700, Ben Gardon wrote:
> > On Fri, Sep 25, 2020 at 6:25 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > >
> > > On 25/09/20 23:23, Ben Gardon wrote:
> > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > index 42dde27decd75..c07831b0c73e1 100644
> > > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > > @@ -124,6 +124,18 @@ static struct kvm_mmu_page *find_tdp_mmu_root_with_role(
> > > > return NULL;
> > > > }
> > > >
> > > > +hpa_t kvm_tdp_mmu_root_hpa_for_role(struct kvm *kvm,
> > > > + union kvm_mmu_page_role role)
> > > > +{
> > > > + struct kvm_mmu_page *root;
> > > > +
> > > > + root = find_tdp_mmu_root_with_role(kvm, role);
> > > > + if (root)
> > > > + return __pa(root->spt);
> > > > +
> > > > + return INVALID_PAGE;
> > > > +}
> > > > +
> > > > static union kvm_mmu_page_role page_role_for_level(struct kvm_vcpu *vcpu,
> > > > int level)
> > > > {
> > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > > > index cc0b7241975aa..2395ffa71bb05 100644
> > > > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > > > @@ -9,6 +9,8 @@ void kvm_mmu_init_tdp_mmu(struct kvm *kvm);
> > > > void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm);
> > > >
> > > > bool is_tdp_mmu_root(struct kvm *kvm, hpa_t root);
> > > > +hpa_t kvm_tdp_mmu_root_hpa_for_role(struct kvm *kvm,
> > > > + union kvm_mmu_page_role role);
> > > > hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
> > > > void kvm_tdp_mmu_put_root_hpa(struct kvm *kvm, hpa_t root_hpa);
> > > >
> > >
> > > Probably missing a piece since this code is not used and neither is the
> > > new argument to is_root_usable.
> > >
> > > I'm a bit confused by is_root_usable since there should be only one PGD
> > > for the TDP MMU (the one for the root_mmu).
> >
> > *facepalm* sorry about that. This commit used to be titled "Implement
> > fast CR3 switching for the TDP MMU" but several refactors later most
> > of it was not useful. The only change that should be part of this
> > patch is the one to avoid clearing the write flooding counts. I must
> > have failed to revert the other changes.
>
> Tangentially related, isn't it possible to end up with multiple roots if the
> MAXPHYSADDR is different between vCPUs? I.e. if userspace coerces KVM into
> using a mix of 4-level and 5-level EPT?
>
> Not saying that's a remotely valid config...
We'll also end up with multiple TDP MMU roots if using SMM, and being
able to switch back and forth between "legacy/shadow MMU" roots and
TDP MMU roots improves nested performance since we can use the TDP MMU
for L1.
Since the TDP MMU associates struct kvm_mmu_pages with all its roots,
no special casing should be needed for root switching.
At one point in this patch set I was using some alternative data
structure to replace struct kvm_mmu_page for the TDP MMU, but I
abandoned that approach.
