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

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Message-ID: <aW4DXajAzC9nn3aJ@yzhao56-desk.sh.intel.com>
Date: Mon, 19 Jan 2026 18:11:41 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "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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	<david@...nel.org>, "michael.roth@....com" <michael.roth@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Peng, Chao P"
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	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 11/24] KVM: x86/mmu: Introduce
 kvm_split_cross_boundary_leafs()

On Mon, Jan 19, 2026 at 04:49:58PM +0800, Huang, Kai wrote:
> On Mon, 2026-01-19 at 08:35 +0000, Huang, Kai wrote:
> > 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.
Hmm, the caller can only figure out when splitting is NOT necessary, e.g., if
start is 1GB-aligned, then there's no need to split for start. However, if start
is not 1GB/2MB-aligned, the caller has no idea if there's a 2MB mapping covering
start - 1 and start.
(for non-TDX cases, if start is not 1GB-aligned and is just 2MB-aligned,
invoking tdp_mmu_split_huge_pages_root() is still necessary because there may
exist a 1GB mapping covering start -1 and start).

In my reply to [*], I didn't want to do the calculation because I didn't see
much overhead from always invoking tdp_mmu_split_huge_pages_root().
But the scenario Sean pointed out is different. When both start and end are not
2MB-aligned, if [start, end) covers a huge range, we can still pre-calculate to
reduce the iterations in tdp_mmu_split_huge_pages_root().

Opportunistically, optimization to skip splits for 1GB-aligned start or end is
possible :)

> > [*]:
> > https://lore.kernel.org/all/35fd7d70475d5743a3c45bc5b8118403036e439b.camel@intel.com/
> 
> Hmm.. thinking again, if you have multiple places needing to do this, then
> kvm_split_cross_boundary_leafs() may serve as a helper to calculate the
> ranges to split.
Yes.