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

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Message-ID: <aXtz37RP2fHIgjLi@google.com>
Date: Thu, 29 Jan 2026 06:51:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Vishal Annapurve <vannapurve@...gle.com>, Kai Huang <kai.huang@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, Fan Du <fan.du@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Dave Hansen <dave.hansen@...el.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>, 
	"vbabka@...e.cz" <vbabka@...e.cz>, "tabba@...gle.com" <tabba@...gle.com>, "david@...nel.org" <david@...nel.org>, 
	"kas@...nel.org" <kas@...nel.org>, "michael.roth@....com" <michael.roth@....com>, Ira Weiny <ira.weiny@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, Chao P Peng <chao.p.peng@...el.com>, 
	"francescolavra.fl@...il.com" <francescolavra.fl@...il.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	Rick P Edgecombe <rick.p.edgecombe@...el.com>, Jun Miao <jun.miao@...el.com>, 
	"jgross@...e.com" <jgross@...e.com>, "pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 11/24] KVM: x86/mmu: Introduce kvm_split_cross_boundary_leafs()

On Thu, Jan 22, 2026, Yan Zhao wrote:
> On Tue, Jan 20, 2026 at 10:02:41AM -0800, Sean Christopherson wrote:
> > On Tue, Jan 20, 2026, Vishal Annapurve wrote:
> > > On Fri, Jan 16, 2026 at 3:39 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > > And then for the PUNCH_HOLE case, do the math to determine which, if any, head
> > > > and tail pages need to be split, and use the existing APIs to make that happen.
> > > 
> > > Just a note: Through guest_memfd upstream syncs, we agreed that
> > > guest_memfd will only allow the punch_hole operation for huge page
> > > size-aligned ranges for hugetlb and thp backing. i.e. the PUNCH_HOLE
> > > operation doesn't need to split any EPT mappings for foreseeable
> > > future.
> > 
> > Oh!  Right, forgot about that.  It's the conversion path that we need to sort out,
> > not PUNCH_HOLE.  Thanks for the reminder!
> Hmm, I see.
> However, do you think it's better to leave the splitting logic in PUNCH_HOLE as
> well? e.g., guest_memfd may want to map several folios in a mapping in the
> future, i.e., after *max_order > folio_order(folio);

No, not at this time.  That is a _very_ big "if".  Coordinating and tracking
contiguous chunks of memory at a larger granularity than the underlying HugeTLB
page size would require significant complexity, I don't see us ever doing that.