Re: [PATCH 07/17] KVM: x86/tdp_mmu: Centralize updates to present external PTEs

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 02, 2026, Rick P Edgecombe wrote:
> On Thu, 2026-04-02 at 09:59 +0800, Yan Zhao wrote:
> > On Thu, Apr 02, 2026 at 07:45:54AM +0800, Edgecombe, Rick P wrote:
> > > On Mon, 2026-03-30 at 14:14 +0800, Yan Zhao wrote:
> > > > > +	KVM_MMU_WARN_ON(is_frozen_spte(new_spte));
> > > > > +
> > > > > +	/*
> > > > > +	 * Temporarily freeze the SPTE until the external PTE operation has
> > > > > +	 * completed (unless the new SPTE itself will be frozen), e.g. so
> > > > > that
> > > > > +	 * concurrent faults don't attempt to install a child PTE in the
> > > > > +	 * external page table before the parent PTE has been written, or
> > > > > try
> > > > > +	 * to re-install a page table before the old one was removed.
> > > > > +	 */
> > > > > +	if (is_mirror_sptep(iter->sptep))
> > > > > +		ret = __tdp_mmu_set_spte_atomic(kvm, iter, FROZEN_SPTE);
> > > > > +	else
> > > > > +		ret = __tdp_mmu_set_spte_atomic(kvm, iter, new_spte);
> > > > >   	if (ret)
> > > > >   		return ret;
> > > > >   
> > > > > -	handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
> > > > > -			    new_spte, iter->level, true);
> > > > > +	ret = __handle_changed_spte(kvm, sp, iter->gfn, iter->old_spte,
> > > > > +				    new_spte, iter->level, true);
> > > > 
> > > > What about adding a comment for the tricky part for the mirror page table:
> > > > while new_spte is set to FROZEN_SPTE in the above __tdp_mmu_set_spte_atomic()
> > > 
> > > You meant it sets iter->sptep I think.
> > > 
> > > > for freezing the mirror page table, the original new_spte from the caller of
> > > > tdp_mmu_set_spte_atomic() is passed to __handle_changed_spte() in order to
> > > > properly update statistics and propagate to the external page table.
> > > 
> > > new_spte was already passed in. What changed? You mean that
> > > __tdp_mmu_set_spte_atomic() sets iter->sptep and doesn't update new_spte? If so
> > > I'm not sure if it threshold TDP MMU.
> > 
> > For mirror page table, a successful path in tdp_mmu_set_spte_atomic() looks
> > like this:
> > tdp_mmu_set_spte_atomic() {
> >   __tdp_mmu_set_spte_atomic(kvm, iter, FROZEN_SPTE); ==>sets mirror to frozen
> >   __handle_changed_spte(kvm, sp, iter->gfn, iter->old_spte,
> > 			new_spte, iter->level, true);==>sets S-EPT to new_spte
> >   __kvm_tdp_mmu_write_spte(iter->sptep, new_spte);   ==>sets mirror to new_spte
> > }
> > 
> > So, the tricky part is that S-EPT is updated to new_spte ahead of mirror EPT.
> 
> I still don't see the point. That ordering is not new, and this patch actually
> adds a bunch of comments around the operations above and below the
> __handle_changed_spte() call. If you think something is still missing maybe you
> can suggest something.

Ya, I'm a bit confused too.  For me, the "tricky" part is understanding the need
to set the mirror SPTE to FROZE_SPTE while updating the external SPTE.  Once that
is understood, I don't find passing in @new_spte to be surprising in any way.

> > > > > @@ -1373,6 +1396,9 @@ static void kvm_tdp_mmu_age_spte(struct kvm *kvm,
> > > > > struct tdp_iter *iter)
> > > > >   {
> > > > >   	u64 new_spte;
> > > > >   
> > > > > +	if (WARN_ON_ONCE(is_mirror_sptep(iter->sptep)))
> > > > > +		return;
> > > > > +
> > > > Add a comment for why mirror page table is not expected here?
> > > 
> > > Ehh, maybe. Thinking about what to put... The warning is kind of cheating a
> > > little bit on the idea of the patch: to forward all changes through limited ops
> > > in a central place, such that we don't have TDX specifics encoded in core MMU.
> > > Trying to forward this through properly would result in more burden to the TDP
> > > MMU, so that's not the right answer either.
> > > 
> > > "Mirror TDP doesn't support PTE aging" is a pretty obvious comment. I'm fine
> > > just leaving it without comment, but I can add something like that. Or do you
> > > have another suggestion?
> > What about "External TDP doesn't support clearing PTE A/D bit"?
> 
> It sounds too close to "TDX doesn't support..." to me. I think I'd prefer to not
> add a comment unless you strongly object.

How about something like this?

	/* TODO: Add support for aging external SPTEs, if necessary. */

That makes it clear that this path is supposed to be unreachable because KVM doesn't
yet support aging external SPTEs, while not trying to say anything about *why*
KVM doesn't support aging external SPTEs.