Re: [PATCH 17/15] KVM: X86: Ensure pae_root to be reconstructed for shadow paging if the guest PDPTEs is changed

Sean Christopherson
2021-12-08T15:29:50Z

On Wed, Dec 08, 2021, Lai Jiangshan wrote:
>
>
> On 2021/12/8 08:15, Sean Christopherson wrote:
> > >
> > > The commit 21823fbda552("KVM: x86: Invalidate all PGDs for the current
> > > PCID on MOV CR3 w/ flush") skips kvm_mmu_free_roots() after
> > > load_pdptrs() when rewriting the CR3 with the same value.
> >
> > This isn't accurate, prior to that commit KVM wasn't guaranteed to do
> > kvm_mmu_free_roots() if it got a hit on the current CR3 or if a previous CR3 in
> > the cache matched the new CR3 (the "cache" has done some odd things in the past).
> >
> > So I think this particular flavor would be:
> >
> > Fixes: 7c390d350f8b ("kvm: x86: Add fast CR3 switch code path")
>
> If guest is 32bit, fast_cr3_switch() always return false, and
> kvm_mmu_free_roots() is always called, and no cr3 goes in prev_root.

Oh, duh, PAE paging.

> > > + /*
> > > + * Ensure the dirty PDPTEs to be loaded for VMX with EPT
> > > + * enabled or pae_root to be reconstructed for shadow paging.
> > > + */
> > > + if (tdp_enabled)
> > > + kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
> > > + else
> > > + kvm_mmu_free_roots(vcpu, vcpu->arch.mmu, KVM_MMU_ROOT_CURRENT);
> >
> > Shouldn't matter since it's legacy shadow paging, but @mmu should be used instead
> > of vcpu->arch.mmuvcpu->arch.mmu.
>
> @mmu is the "guest mmu" (vcpu->arch.walk_mmu), which is used to walk
> including loading pdptr.
>
> vcpu->arch.mmu is for host constructing mmu for shadowed or tdp mmu
> which is used in host side management including kvm_mmu_free_roots().
>
> Even they are the same pointer now for !tdp, the meaning is different. I prefer
> to distinguish them even before kvm_mmu is split different for guest mmu
> (vcpu->arch.walk_mmu) and host constructing mmu (vcpu->arch.mmu).

I see where you're coming from. I was viewing it from the perspective of,
"they're the same thing for shadow paging, why reread mmu?".

I'm ok with explicitly referencing arch.mmu, but maybe add a comment?