linux-kernel - [RFC PATCH v2 07/23] KVM: x86/mmu: Disallow page merging (huge page adjustment) for mirror root

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Message-ID: <20250807094254.4537-1-yan.y.zhao@intel.com>
Date: Thu,  7 Aug 2025 17:42:54 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	x86@...nel.org,
	rick.p.edgecombe@...el.com,
	dave.hansen@...el.com,
	kas@...nel.org,
	tabba@...gle.com,
	ackerleytng@...gle.com,
	quic_eberman@...cinc.com,
	michael.roth@....com,
	david@...hat.com,
	vannapurve@...gle.com,
	vbabka@...e.cz,
	thomas.lendacky@....com,
	pgonda@...gle.com,
	zhiquan1.li@...el.com,
	fan.du@...el.com,
	jun.miao@...el.com,
	ira.weiny@...el.com,
	isaku.yamahata@...el.com,
	xiaoyao.li@...el.com,
	binbin.wu@...ux.intel.com,
	chao.p.peng@...el.com,
	yan.y.zhao@...el.com
Subject: [RFC PATCH v2 07/23] KVM: x86/mmu: Disallow page merging (huge page adjustment) for mirror root

From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>

Disallow page merging (huge page adjustment) for the mirror root by
utilizing disallowed_hugepage_adjust().

Make the mirror root check asymmetric with NX huge pages and not to litter
the generic MMU code:

Invoke disallowed_hugepage_adjust() in kvm_tdp_mmu_map() when necessary,
specifically when KVM has mirrored TDP or the NX huge page workaround is
enabled.

Check and reduce the goal_level of a fault internally in
disallowed_hugepage_adjust() when the fault is for a mirror root and
there's a shadow present non-leaf entry at the original goal_level.

Signed-off-by: Edgecombe, Rick P <rick.p.edgecombe@...el.com>
Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
RFC v2:
- Check is_mirror_sp() in disallowed_hugepage_adjust() instead of passing
  in an is_mirror arg. (Rick)
- Check kvm_has_mirrored_tdp() in kvm_tdp_mmu_map() to determine whether
  to invoke disallowed_hugepage_adjust(). (Rick)

RFC v1:
- new patch
---
 arch/x86/kvm/mmu/mmu.c     | 3 ++-
 arch/x86/kvm/mmu/tdp_mmu.c | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 3f76415cec71..9182192daa3a 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3412,7 +3412,8 @@ 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 bb95c95f6531..f9a054754544 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1243,6 +1243,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	struct tdp_iter iter;
 	struct kvm_mmu_page *sp;
 	int ret = RET_PF_RETRY;
+	bool hugepage_adjust_disallowed = fault->nx_huge_page_workaround_enabled ||
+					  kvm_has_mirrored_tdp(kvm);
 
 	kvm_mmu_hugepage_adjust(vcpu, fault);
 
@@ -1253,7 +1255,7 @@ 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)
+		if (hugepage_adjust_disallowed)
 			disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
 
 		/*
-- 
2.43.2