Re: [PATCH 12/12] KVM: x86: do not unload MMU roots on all role changes

[Paolo Bonzini]

On 2/14/22 20:24, Sean Christopherson wrote:
> On Mon, Feb 14, 2022, Paolo Bonzini wrote:
> > On 2/11/22 19:48, Sean Christopherson wrote:
> > > On Wed, Feb 09, 2022, Paolo Bonzini wrote:
> > > > -	kvm_mmu_unload(vcpu);
> > > >    	kvm_init_mmu(vcpu);
> > > > +	kvm_mmu_new_pgd(vcpu, vcpu->arch.cr3);
> > >
> > > This is too risky IMO, there are far more flows than just MOV CR0/CR4 that are
> > > affected, e.g. SMM transitions, KVM_SET_SREG, etc...
>
> I'm not concerned about the TLB flush aspects so much as the addition of
> kvm_mmu_new_pgd() in new paths.

Okay, yeah those are more complex and the existing ones are broken too.

> > > > -	if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS)
> > > > +	if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS) {
> > > > +		/* Flush the TLB if CR0 is changed 1 -> 0.  */
> > > > +		if ((old_cr0 & X86_CR0_PG) && !(cr0 & X86_CR0_PG))
> > > > +			kvm_mmu_unload(vcpu);
> > >
> > > Calling kvm_mmu_unload() instead of requesting a flush isn't coherent with respect
> > > to the comment, or with SMEP handling.  And the SMEP handling isn't coherent with
> > > respect to the changelog.  Please elaborate :-)
> >
> > Yep, will do (the CR0.PG=0 case is similar to the CR0.PCIDE=0 case below).
>
> Oh, you're freeing all roots to ensure a future MOV CR3 with NO_FLUSH and PCIDE=1
> can't reuse a stale root.  That's necessary if and only if the MMU is shadowing
> the guest, non-nested TDP MMUs just need to flush the guest's TLB.  The same is
> true for the PCIDE case, i.e. we could optimize that too, though the main motivation
> would be to clarify why all roots are unloaded.

Yes.  Clarifying all this should be done before the big change to 
kvm_mmu_reset_context().

> > Using kvm_mmu_unload() avoids loading a cached root just to throw it away
> > immediately after,
>
> The shadow paging case will throw it away, but not the non-nested TDP MMU case?

Yes, the TDP case is okay since the role is the same.  kvm_init_mmu() is 
enough.

> > but I can change this to a new KVM_REQ_MMU_UPDATE_ROOT flag that does
> >
> > 	kvm_mmu_new_pgd(vcpu, vcpu->arch.cr3);
>
> I don't think that's necessary, I was just confused by the discrepancy.

It may not be necessary but it is clearer IMO.  Let me post a new patch.

Paolo