Re: [PATCH 5/5] KVM: x86/mmu: Use dummy root, backed by zero page, for !visible guest roots

[Yu Zhang]

On Tue, Jul 25, 2023 at 08:53:19AM -0700, Sean Christopherson wrote:
> On Tue, Jul 25, 2023, Yu Zhang wrote:
> > > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > > index 122bfc0124d3..e9d4d7b66111 100644
> > > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > > @@ -646,6 +646,17 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > >  	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> > >  		goto out_gpte_changed;
> > >  
> > > +	/*
> > > +	 * Load a new root and retry the faulting instruction in the extremely
> > > +	 * unlikely scenario that the guest root gfn became visible between
> > > +	 * loading a dummy root and handling the resulting page fault, e.g. if
> > > +	 * userspace create a memslot in the interim.
> > > +	 */
> > > +	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
> > > +		kvm_mmu_unload(vcpu);
> > 
> > Do we really need a kvm_mmu_unload()? Could we just set
> > vcpu->arch.mmu->root.hpa to INVALID_PAGE here?
> 
> Oof, yeah.  Not only is a full unload overkill, if this code were hit it would
> lead to deadlock because kvm_mmu_free_roots() expects to be called *without*
> mmu_lock held.
> 
> Hmm, but I don't love the idea of open coding a free/reset of the current root.
> I'm leaning toward
> 
> 		kvm_make_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu);
> 
> since it's conceptually similar to KVM unloading roots when a memslot is deleted
> or moved, just reversed.  That would obviously tie this code to KVM's handling of
> the dummy root just as much as manually invalidating root.hpa (probably more so),
> but that might actually be a good thing because then the rule for the dummy root
> is that it's always considered obsolete (when checked), and that could be
> explicitly documented in is_obsolete_root().
> 

Oh, right. KVM_REQ_MMU_FREE_OBSOLETE_ROOTS should work. Thanks!

B.R.
Yu