linux-kernel - Re: [PATCH v3 11/24] KVM: x86/mmu: Introduce kvm_split_cross_boundary

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Message-ID: <2906b4d3b789985917a063d095c4063ee6ab7b72.camel@intel.com>
Date: Thu, 15 Jan 2026 12:25:21 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Du, Fan" <fan.du@...el.com>,
	"Li, Xiaoyao" <xiaoyao.li@...el.com>, "Gao, Chao" <chao.gao@...el.com>,
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	<thomas.lendacky@....com>, "vbabka@...e.cz" <vbabka@...e.cz>,
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 Chao P" <chao.p.peng@...el.com>, "francescolavra.fl@...il.com"
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Subject: Re: [PATCH v3 11/24] KVM: x86/mmu: Introduce
 kvm_split_cross_boundary_leafs()

On Tue, 2026-01-06 at 18:21 +0800, Yan Zhao wrote:
> @@ -1692,12 +1707,35 @@ void kvm_tdp_mmu_try_split_huge_pages(struct kvm *kvm,
>  
>  	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
>  	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, slot->as_id) {
> -		r = tdp_mmu_split_huge_pages_root(kvm, root, start, end, target_level, shared);
> +		r = tdp_mmu_split_huge_pages_root(kvm, root, start, end, target_level,
> +						  shared, false);
> +		if (r) {
> +			kvm_tdp_mmu_put_root(kvm, root);
> +			break;
> +		}
> +	}
> +}
> +
> +int kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs(struct kvm *kvm,
> +						     struct kvm_gfn_range *range,
> +						     bool shared)
> +{
> +	enum kvm_tdp_mmu_root_types types;
> +	struct kvm_mmu_page *root;
> +	int r = 0;
> +
> +	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> +	types = kvm_gfn_range_filter_to_root_types(kvm, range->attr_filter);
> +
> +	__for_each_tdp_mmu_root_yield_safe(kvm, root, range->slot->as_id, types) {
> +		r = tdp_mmu_split_huge_pages_root(kvm, root, range->start, range->end,
> +						  PG_LEVEL_4K, shared, true);
>  		if (r) {
>  			kvm_tdp_mmu_put_root(kvm, root);
>  			break;
>  		}
>  	}
> +	return r;
>  }
>  

Seems the two functions -- kvm_tdp_mmu_try_split_huge_pages() and
kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs() -- are almost
identical.  Is it better to introduce a helper and make the two just be
the wrappers?

E.g.,

static int __kvm_tdp_mmu_split_huge_pages(struct kvm *kvm, 
					  struct kvm_gfn_range *range,
					  int target_level,
					  bool shared,
					  bool cross_boundary_only)
{
	...
}

And by using this helper, I found the name of the two wrapper functions
are not ideal:

kvm_tdp_mmu_try_split_huge_pages() is only for log dirty, and it should
not be reachable for TD (VM with mirrored PT).  But currently it uses
KVM_VALID_ROOTS for root filter thus mirrored PT is also included.  I
think it's better to rename it, e.g., at least with "log_dirty" in the
name so it's more clear this function is only for dealing log dirty (at
least currently).  We can also add a WARN() if it's called for VM with
mirrored PT but it's a different topic.

kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs() doesn't have
"huge_pages", which isn't consistent with the other.  And it is a bit
long.  If we don't have "gfn_range" in __kvm_tdp_mmu_split_huge_pages(),
then I think we can remove "gfn_range" from
kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs() too to make it shorter.

So how about:

Rename kvm_tdp_mmu_try_split_huge_pages() to
kvm_tdp_mmu_split_huge_pages_log_dirty(), and rename
kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs() to
kvm_tdp_mmu_split_huge_pages_cross_boundary()

?

E.g.,:

int kvm_tdp_mmu_split_huge_pages_log_dirty(struct kvm *kvm, 
				     	   const kvm_memory_slot *slot,
				    	   gfn_t start, gfn_t end,
					   int target_level, bool shared)
{
	struct kvm_gfn_range range = {
		.slot		= slot,
		.start		= start,
		.end		= end,
		.attr_filter	= 0, /* doesn't matter */
		.may_block	= true,
	};

	if (WARN_ON_ONCE(kvm_has_mirrored_tdp(kvm))
		return -EINVAL;

	return __kvm_tdp_mmu_split_huge_pages(kvm, &range, target_level,
					      shared, false);
}

int kvm_tdp_mmu_split_huge_pages_cross_boundary(struct kvm *kvm,
					struct kvm_gfn_range *range,
					int target_level,
					bool shared)
{
	return __kvm_tdp_mmu_split_huge_pages(kvm, range, target_level,
					      shared, true);
}

Anything I missed?

And one more minor thing:

With that, I think you can move range->may_block check from
kvm_split_cross_boundary_leafs() to the __kvm_tdp_mmu_split_huge_pages()
common helper:

	if (!range->may_block)
		return -EOPNOTSUPP;