Re: [PATCH v4 05/17] KVM: x86/tdp_mmu: Alloc external_spt page for mirror page table splitting

[Edgecombe, Rick P]

On Mon, 2026-09-28 at 17:09 +0800, Yan Zhao wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> Enhance tdp_mmu_alloc_sp_for_split() to allocate a page table page for the
> external page table in preparation for splitting the mirror page table.
> 
> When the mirror page table is split in tdp_mmu_split_huge_page(), the
> corresponding external page table also needs to be split. Therefore,
> allocate external_spt in tdp_mmu_alloc_sp_for_split() to prepare for the
> splitting.
> 
> The external_spt will be gifted to the TDX module and mapped as a page
> table page in the S-EPT during splitting. Since the TDX module will
> initialize the page content in the DEMOTE SEAMCALL, there is no need to
> zero external_spt.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> [sean: use __get_free_page(), let is_mirror_root be const and called once]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> ---
> v4:
> - "bool mirror" --> "bool is_mirror_sp". (Sean)
> - Use __get_free_page() instead of get_zeroed_page(). (Sean)
> - let is_mirror_root be const and called once. (Sean)
> 
> v3:
> - Removed unnecessary declaration of tdp_mmu_alloc_sp_for_split(). (Kai)
> - Fixed a typo in the patch log. (Kai)
> 
> RFC v2:
> - NO change.
> 
> RFC v1:
> - Rebased and simplified the code.
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 001449142d32..f3311317a63a 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1466,7 +1466,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
>  	return spte_set;
>  }
>  
> -static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(void)
> +static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(bool is_mirror_sp)
>  {
>  	struct kvm_mmu_page *sp;
>  
> @@ -1480,6 +1480,15 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(void)
>  		return NULL;
>  	}
>  
> +	if (is_mirror_sp) {
> +		sp->external_spt = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
> +		if (!sp->external_spt) {
> +			free_page((unsigned long)sp->spt);
> +			kmem_cache_free(mmu_page_header_cache, sp);
> +			return NULL;
> +		}
> +	}
> +
>  	return sp;
>  }
>  
> @@ -1527,6 +1536,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
>  					 gfn_t start, gfn_t end,
>  					 int target_level, bool shared)
>  {
> +	const bool is_mirror_root = is_mirror_sp(root);

Nit:

is_mirror_root and is_mirror_sp(root) almost even read the same. And the local
var is only every used once even at the end of this series. I wonder about just
having tdp_mmu_alloc_sp_for_split(is_mirror_sp(root)), or even just passing the
root in.

Otherwise:

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

>  	struct kvm_mmu_page *sp = NULL;
>  	struct tdp_iter iter;
>  
> @@ -1559,7 +1569,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
>  			else
>  				write_unlock(&kvm->mmu_lock);
>  
> -			sp = tdp_mmu_alloc_sp_for_split();
> +			sp = tdp_mmu_alloc_sp_for_split(is_mirror_root);
>  
>  			if (shared)
>  				read_lock(&kvm->mmu_lock);