Re: [PATCH 2/4] KVM: x86/mmu: Harden "map private PFN" against unexpected root invalidation

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 07, 2026, Rick P Edgecombe wrote:
> On Thu, 2026-08-06 at 14:40 -0700, Sean Christopherson wrote:
> > @@ -5199,10 +5195,21 @@ int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
> >  		if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu))
> >  			return -EIO;
> >  
> > +		r = kvm_mmu_reload(vcpu);
> > +		if (r)
> > +			return r;
> > +
> >  		cond_resched();
> >  
> >  		guard(read_lock)(&kvm->mmu_lock);
> >  
> > +		/*
> > +		 * Because slots_lock is held, it should be impossible for roots
> > +		 * to be invalidated after the initial MMU reload.  WARN, but
> > +		 * continue and re-reload the MMU to try and keep the VM alive.
> > +		 */
> 
> It might be a little confusing when no MMU reload follows the comment. Maybe
> mention that the MMU reload is above? Either way ok to me.

Yeah, I don't love the comment either.  The subtlety that it doesn't capture is
that KVM_REQ_MMU_FREE_OBSOLETE_ROOTS doesn't actually mean the current root is
invalid, just that it might be invalid and the vCPU needs to check.

How about this?  

		/*
		 * Because slots_lock is held, it should be impossible for *any*
		 * roots to be invalidated after the initial MMU reload.  WARN,
		 * but continue on; the above MMU reload will do the right thing
		 * if the current root is actually invalid.
		 */

> > +		WARN_ON_ONCE(kvm_test_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu));
> > +
> >  		r = kvm_tdp_mmu_map(vcpu, &fault);
>