Re: [RFC PATCH v2 12/23] KVM: x86/mmu: Introduce kvm_split_cross_boundary_leafs()

[Huang, Kai]

On Thu, 2025-08-07 at 17:43 +0800, Yan Zhao wrote:
>  static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
>  					 struct kvm_mmu_page *root,
>  					 gfn_t start, gfn_t end,
> -					 int target_level, bool shared)
> +					 int target_level, bool shared,
> +					 bool only_cross_bounday, bool *flush)
>  {
>  	struct kvm_mmu_page *sp = NULL;
>  	struct tdp_iter iter;
> @@ -1589,6 +1596,13 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
>  	 * level into one lower level. For example, if we encounter a 1GB page
>  	 * we split it into 512 2MB pages.
>  	 *
> +	 * When only_cross_bounday is true, just split huge pages above the
> +	 * target level into one lower level if the huge pages cross the start
> +	 * or end boundary.
> +	 *
> +	 * No need to update @flush for !only_cross_bounday cases, which rely
> +	 * on the callers to do the TLB flush in the end.
> +	 *

s/only_cross_bounday/only_cross_boundary

From tdp_mmu_split_huge_pages_root()'s perspective, it's quite odd to only
update 'flush' when 'only_cross_bounday' is true, because
'only_cross_bounday' can only results in less splitting.

I understand this is because splitting S-EPT mapping needs flush (at least
before non-block DEMOTE is implemented?).  Would it better to also let the
caller to decide whether TLB flush is needed?  E.g., we can make
tdp_mmu_split_huge_pages_root() return whether any split has been done or
not.  I think this should also work?