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

[Paolo Bonzini]

On 12/8/21 01:15, Sean Christopherson wrote:
> > @@ -832,8 +832,14 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
> >   	if (memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs))) {
> >   		memcpy(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs));
> >   		kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
> > -		/* Ensure the dirty PDPTEs to be loaded. */
> > -		kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
> > +		/*
> > +		 * 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.

In kvm/next actually there's no mmu parameter to load_pdptrs, so it's 
okay to keep vcpu->arch.mmu.

> To avoid a dependency on the previous patch, I think it makes sense to have this be:
>
> 	if (!tdp_enabled && memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs)))
> 		kvm_mmu_free_roots(vcpu, mmu, KVM_MMU_ROOT_CURRENT);
>
> before the memcpy().
>
> Then we can decide independently if skipping the KVM_REQ_LOAD_MMU_PGD if the
> PDPTRs are unchanged with respect to the MMU is safe.

Do you disagree that there's already an invariant that the PDPTRs can 
only be dirty if KVM_REQ_LOAD_MMU_PGD---and therefore a previous change 
to the PDPTRs would have triggered KVM_REQ_LOAD_MMU_PGD?  This is 
opposed to the guest TLB flush due to MOV CR3; that one has to be done 
unconditionally for PAE paging, and it is handled separately within 
kvm_set_cr3.

Paolo