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

[Yan Zhao]

On Fri, Feb 06, 2026 at 08:09:06AM -0800, Sean Christopherson wrote:
> > So, it's incorrect to invoke is_mirror_sptep() which internally contains
> > rcu_dereference(), resulting in "WARNING: suspicious RCU usage".
> 
> Ah, now I see why the previous code pass in a bool.  I don't love passing a bool,
> but passing @iter is outright dangerous, so I guess this?

LGTM.

> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index a32192c35099..4d92c0d19d7c 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1448,7 +1448,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
>  }
>  
>  static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
> -                                                      struct tdp_iter *iter)
> +                                                      bool is_mirror_sp)
>  {
>         struct kvm_mmu_page *sp;
>  
> @@ -1460,7 +1460,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
>         if (!sp->spt)
>                 goto err_spt;
>  
> -       if (is_mirror_sptep(iter->sptep)) {
> +       if (is_mirror_sp) {
>                 sp->external_spt = (void *)kvm_x86_call(alloc_external_sp)(GFP_KERNEL_ACCOUNT);
>                 if (!sp->external_spt)
>                         goto err_external_spt;
> @@ -1525,6 +1525,7 @@ 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);
>         struct kvm_mmu_page *sp = NULL;
>         struct tdp_iter iter;
>  
> @@ -1557,7 +1558,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
>                         else
>                                 write_unlock(&kvm->mmu_lock);
>  
> -                       sp = tdp_mmu_alloc_sp_for_split(kvm, &iter);
> +                       sp = tdp_mmu_alloc_sp_for_split(kvm, is_mirror_root);
>  
>                         if (shared)
>                                 read_lock(&kvm->mmu_lock);