Re: [PATCH 06/17] KVM: x86/tdp_mmu: Morph the !is_frozen_spte() check into a KVM_MMU_WARN_ON()

[Yan Zhao]

On Fri, Mar 27, 2026 at 01:14:10PM -0700, Rick Edgecombe wrote:
> Remove the conditional logic for handling the setting of mirror EPTs to
Should we unify the terms "mirror EPTs," "mirror TDP," and "mirror page tables"
in this series?

> frozen in __tdp_mmu_set_spte_atomic() and add it as a warning instead.
> 
> Mirror TDP needs propagate PTE changes to the to the external TDP. This
Two "to the".

> presents a problem for atomic updates which can't update both at once. So
> a special value, FROZEN_SPTE, is used as a temporary state during these
> updates to prevent concurrent operations to the PTE. If the TDP MMU tried
> to install this as a long term value, it would confuse these updates.
> Despite this __tdp_mmu_set_spte_atomic() includes a check to handle it
> being set. Remove this check and turn it into a warning.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 0809fe8e8737..338957bc5109 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -656,7 +656,13 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
>  	 */
>  	WARN_ON_ONCE(iter->yielded || is_frozen_spte(iter->old_spte));
>  
> -	if (is_mirror_sptep(iter->sptep) && !is_frozen_spte(new_spte)) {
> +	/*
> +	 * FROZEN_SPTE is a temporary state and should never be set via higher
> +	 * level helpers.
> +	 */
> +	KVM_MMU_WARN_ON(is_frozen_spte(new_spte));
Why is KVM_MMU_WARN_ON() used here for new_spte while WARN_ON_ONCE() is used
above for old_spte?

> +	if (is_mirror_sptep(iter->sptep)) {
>  		int ret;
>  
>  		ret = set_external_spte_present(kvm, iter->sptep, iter->gfn,
> -- 
> 2.53.0
>