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

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Message-ID: <CAGtprH9OWFJc=_T=rChSjhJ3utPNcV_L_+-zq5uqtmXm-ffgNQ@mail.gmail.com>
Date: Tue, 20 Jan 2026 09:57:32 -0800
From: Vishal Annapurve <vannapurve@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Kai Huang <kai.huang@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Yan Y Zhao <yan.y.zhao@...el.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 Fri, Jan 16, 2026 at 3:39 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Jan 15, 2026, Kai Huang wrote:
> > 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()
> >
> > ?
>
> 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.
>
> And once kvm_arch_pre_set_memory_attributes() is dropped, I'm pretty sure the
> _only_ usage is for guest_memfd PUNCH_HOLE, because unless I'm misreading the
> code, the usage in tdx_honor_guest_accept_level() is superfluous and confusing.
>
> For the EPT violation case, the guest is accepting a page.  Just split to the
> guest's accepted level, I don't see any reason to make things more complicated
> than that.
>
> 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.