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

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Message-ID: <3ef110f63cbbc65d6d4cbf737b26c09cb7b44e7c.camel@intel.com>
Date: Mon, 19 Jan 2026 08:35:26 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
CC: "Du, Fan" <fan.du@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Li, Xiaoyao" <xiaoyao.li@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
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Subject: Re: [PATCH v3 11/24] KVM: x86/mmu: Introduce
 kvm_split_cross_boundary_leafs()

On Mon, 2026-01-19 at 09:28 +0800, Zhao, Yan Y wrote:
> > I find the "cross_boundary" termininology extremely confusing.  I also dislike
> > the concept itself, in the sense that it shoves a weird, specific concept into
> > the guts of the TDP MMU.
> > The other wart is that it's inefficient when punching a large hole.  E.g. say
> > there's a 16TiB guest_memfd instance (no idea if that's even possible), and then
> > userpace punches a 12TiB hole.  Walking all ~12TiB just to _maybe_ split the head
> > and tail pages is asinine.
> That's a reasonable concern. I actually thought about it.
> My consideration was as follows:
> Currently, we don't have such large areas. Usually, the conversion ranges are
> less than 1GB. Though the initial conversion which converts all memory from
> private to shared may be wide, there are usually no mappings at that stage. So,
> the traversal should be very fast (since the traversal doesn't even need to go
> down to the 2MB/1GB level).
> 
> If the caller of kvm_split_cross_boundary_leafs() finds it needs to convert a
> very large range at runtime, it can optimize by invoking the API twice:
> once for range [start, ALIGN(start, 1GB)), and
> once for range [ALIGN_DOWN(end, 1GB), end).
> 
> I can also implement this optimization within kvm_split_cross_boundary_leafs()
> by checking the range size if you think that would be better.

I am not sure why do we even need kvm_split_cross_boundary_leafs(), if you
want to do optimization.

I think I've raised this in v2, and asked why not just letting the caller
to figure out the ranges to split for a given range (see at the end of
[*]), because the "cross boundary" can only happen at the beginning and
end of the given range, if possible.

[*]:
https://lore.kernel.org/all/35fd7d70475d5743a3c45bc5b8118403036e439b.camel@intel.com/