Re: [RFC PATCH v5 37/45] KVM: x86/tdp_mmu: Alloc external_spt page for mirror page table splitting

[Yan Zhao]

On Wed, Jan 28, 2026 at 05:15:09PM -0800, Sean Christopherson 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 for splitting the mirror page table.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> [sean: use kvm_x86_ops.alloc_external_sp()]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 3b0da898824a..4f5b80f0ca03 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1447,7 +1447,7 @@ 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(struct tdp_iter *iter)
>  {
>  	struct kvm_mmu_page *sp;
>  
> @@ -1461,6 +1461,15 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(void)
>  		return NULL;
>  	}
>  
> +	if (is_mirror_sptep(iter->sptep)) {
tdp_mmu_alloc_sp_for_split() is invoked in tdp_mmu_split_huge_pages_root() after
rcu_read_unlock() is called.

So, it's incorrect to invoke is_mirror_sptep() which internally contains
rcu_dereference(), resulting in "WARNING: suspicious RCU usage".

> +		sp->external_spt = (void *)kvm_x86_call(alloc_external_sp)(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;
>  }
>  
> @@ -1540,7 +1549,7 @@ 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(&iter);
>  
>  			if (shared)
>  				read_lock(&kvm->mmu_lock);
> -- 
> 2.53.0.rc1.217.geba53bf80e-goog
>