Re: [PATCH v3 06/24] KVM: x86/mmu: Disallow page merging (huge page adjustment) for mirror root

[Sean Christopherson]

On Fri, Jan 16, 2026, Yan Zhao wrote:
> Hi Sean,
> Thanks for the review!
> 
> On Thu, Jan 15, 2026 at 02:49:59PM -0800, Sean Christopherson wrote:
> > On Tue, Jan 06, 2026, Yan Zhao wrote:
> > > From: Rick P Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > > 
> > > Disallow page merging (huge page adjustment) for the mirror root by
> > > utilizing disallowed_hugepage_adjust().
> > 
> > Why?  What is this actually doing?  The below explains "how" but I'm baffled as
> > to the purpose.  I'm guessing there are hints in the surrounding patches, but I
> > haven't read them in depth, and shouldn't need to in order to understand the
> > primary reason behind a change.
> Sorry for missing the background. I will explain the "why" in the patch log in
> the next version.
> 
> The reason for introducing this patch is to disallow page merging for TDX. I
> explained the reasons to disallow page merging in the cover letter:
> 
> "
> 7. Page merging (page promotion)
> 
>    Promotion is disallowed, because:
> 
>    - The current TDX module requires all 4KB leafs to be either all PENDING
>      or all ACCEPTED before a successful promotion to 2MB. This requirement
>      prevents successful page merging after partially converting a 2MB
>      range from private to shared and then back to private, which is the
>      primary scenario necessitating page promotion.
> 
>    - tdh_mem_page_promote() depends on tdh_mem_range_block() in the current
>      TDX module. Consequently, handling BUSY errors is complex, as page
>      merging typically occurs in the fault path under shared mmu_lock.
> 
>    - Limited amount of initial private memory (typically ~4MB) means the
>      need for page merging during TD build time is minimal.
> "

> However, we currently don't support page merging yet. Specifically for the above
> scenariol, the purpose is to avoid handling the error from
> tdh_mem_page_promote(), which SEAMCALL currently needs to be preceded by
> tdh_mem_range_block(). To handle the promotion error (e.g., due to busy) under
> read mmu_lock, we may need to introduce several spinlocks and guarantees from
> the guest to ensure the success of tdh_mem_range_unblock() to restore the S-EPT
> status. 
> 
> Therefore, we introduced this patch for simplicity, and because the promotion
> scenario is not common.

Say that in the changelog!  Describing the "how" in detail is completely unnecessary,
or at least it should be.  Because I strongly disagree with Rick's opinion from
the RFC that kvm_tdp_mmu_map() should check kvm_has_mirrored_tdp()[*].

 : I think part of the thing that is bugging me is that
 : nx_huge_page_workaround_enabled is not conceptually about whether the specific
 : fault/level needs to disallow huge page adjustments, it's whether it needs to
 : check if it does. Then disallowed_hugepage_adjust() does the actual specific
 : checking. But for the mirror logic the check is the same for both. It's
 : asymmetric with NX huge pages, and just sort of jammed in. It would be easier to
 : follow if the kvm_tdp_mmu_map() conditional checked wither mirror TDP was
 : "active", rather than the mirror role.

[*] http://lore.kernel.org/all/eea0bf7925c3b9c16573be8e144ddcc77b54cc92.camel@xxxxxxxxx

If the changelog explains _why_, and the code is actually commented, then calling
into disallowed_hugepage_adjust() for all faults in a VM with mirrored roots is
nonsensical, because the code won't match the comment.

From: "Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx>
Date: Tue, 22 Apr 2025 10:21:12 +0800
Subject: [PATCH] KVM: x86/mmu: Prevent hugepage promotion for mirror roots in
 fault path

Disallow hugepage promotion in the TDP MMU for mirror roots as KVM doesn't
currently support promoting S-EPT entries due to the complexity incurred
by the TDX-Module's rules for hugepage promotion.

 - The current TDX-Module requires all 4KB leafs to be either all PENDING
   or all ACCEPTED before a successful promotion to 2MB. This requirement
   prevents successful page merging after partially converting a 2MB
   range from private to shared and then back to private, which is the
   primary scenario necessitating page promotion.

 - The TDX-Module effectively requires a break-before-make sequence (to
   satisfy its TLB flushing rules), i.e. creates a window of time where a
   different vCPU can encounter faults on a SPTE that KVM is trying to
   promote to a hugepage.  To avoid unexpected BUSY errors, KVM would need
   to FREEZE the non-leaf SPTE before replacing it with a huge SPTE.

Disable hugepage promotion for all map() operations, as supporting page
promotion when building the initial image is still non-trivial, and the
vast majority of images are ~4MB or less, i.e. the benefit of creating
hugepages during TD build time is minimal.

Signed-off-by: Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx>
Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
[sean: check root, add comment, rewrite changelog]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c     |  3 ++-
 arch/x86/kvm/mmu/tdp_mmu.c | 12 +++++++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4ecbf216d96f..45650f70eeab 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3419,7 +3419,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
 	    cur_level == fault->goal_level &&
 	    is_shadow_present_pte(spte) &&
 	    !is_large_pte(spte) &&
-	    spte_to_child_sp(spte)->nx_huge_page_disallowed) {
+	    ((spte_to_child_sp(spte)->nx_huge_page_disallowed) ||
+	     is_mirror_sp(spte_to_child_sp(spte)))) {
 		/*
 		 * A small SPTE exists for this pfn, but FNAME(fetch),
 		 * direct_map(), or kvm_tdp_mmu_map() would like to create a
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 321dbde77d3f..0fe3be41594f 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1232,7 +1232,17 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	for_each_tdp_pte(iter, kvm, root, fault->gfn, fault->gfn + 1) {
 		int r;
 
-		if (fault->nx_huge_page_workaround_enabled)
+		/*
+		 * Don't replace a page table (non-leaf) SPTE with a huge SPTE
+		 * (a.k.a. hugepage promotion) if the NX hugepage workaround is
+		 * enabled, as doing so will cause significant thrashing if one
+		 * or more leaf SPTEs needs to be executable.
+		 *
+		 * Disallow hugepage promotion for mirror roots as KVM doesn't
+		 * (yet) support promoting S-EPT entries while holding mmu_lock
+		 * for read (due to complexity induced by the TDX-Module APIs).
+		 */
+		if (fault->nx_huge_page_workaround_enabled || is_mirror_sp(root))
 			disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
 
 		/*

base-commit: 914ea33c797e95e5fa7a0803e44b621a9e70a90f
--