Re: [RFC PATCH v2 09/23] KVM: x86/tdp_mmu: Add split_external_spt hook called during write mmu_lock

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/7/2025 5:43 PM, Yan Zhao wrote:
[...]
>   /**
>    * handle_removed_pt() - handle a page table removed from the TDP structure
>    *
> @@ -765,12 +778,20 @@ static u64 tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
>   	handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
>   
>   	/*
> -	 * Users that do non-atomic setting of PTEs don't operate on mirror
> -	 * roots, so don't handle it and bug the VM if it's seen.
> +	 * Propagate changes of SPTE to the external page table under write
> +	 * mmu_lock.
> +	 * Current valid transitions:
> +	 * - present leaf to !present.
> +	 * - present non-leaf to !present.

Nit:
Maybe add a small note to limit the scenario, such as "after releasing the HKID"
or "during the TD teardown"?

> +	 * - present leaf to present non-leaf (splitting)
>   	 */
>   	if (is_mirror_sptep(sptep)) {
> -		KVM_BUG_ON(is_shadow_present_pte(new_spte), kvm);
> -		remove_external_spte(kvm, gfn, old_spte, level);
> +		if (!is_shadow_present_pte(new_spte))
> +			remove_external_spte(kvm, gfn, old_spte, level);
> +		else if (is_last_spte(old_spte, level) && !is_last_spte(new_spte, level))
> +			split_external_spt(kvm, gfn, old_spte, new_spte, level);
> +		else
> +			KVM_BUG_ON(1, kvm);
>   	}
>   
>   	return old_spte;