linux-kernel - [RFC PATCH v5 06/45] KVM: x86/mmu: Fold set_external_spte

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Message-ID: <20260129011517.3545883-7-seanjc@google.com>
Date: Wed, 28 Jan 2026 17:14:38 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Kiryl Shutsemau <kas@...nel.org>, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	kvm@...r.kernel.org, Kai Huang <kai.huang@...el.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>, 
	Sagi Shahar <sagis@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: [RFC PATCH v5 06/45] KVM: x86/mmu: Fold set_external_spte_present()
 into its sole caller

Fold set_external_spte_present() into __tdp_mmu_set_spte_atomic() in
anticipation of supporting hugepage splitting, at which point other paths
will also set shadow-present external SPTEs.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 82 +++++++++++++++++---------------------
 1 file changed, 36 insertions(+), 46 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 56ad056e6042..6fb48b217f5b 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -495,33 +495,6 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
 	call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
 }
 
-static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t sptep,
-						 gfn_t gfn, u64 *old_spte,
-						 u64 new_spte, int level)
-{
-	int ret;
-
-	lockdep_assert_held(&kvm->mmu_lock);
-
-	if (KVM_BUG_ON(is_shadow_present_pte(*old_spte), kvm))
-		return -EIO;
-
-	/*
-	 * We need to lock out other updates to the SPTE until the external
-	 * page table has been modified. Use FROZEN_SPTE similar to
-	 * the zapping case.
-	 */
-	if (!try_cmpxchg64(rcu_dereference(sptep), old_spte, FROZEN_SPTE))
-		return -EBUSY;
-
-	ret = kvm_x86_call(set_external_spte)(kvm, gfn, level, new_spte);
-	if (ret)
-		__kvm_tdp_mmu_write_spte(sptep, *old_spte);
-	else
-		__kvm_tdp_mmu_write_spte(sptep, new_spte);
-	return ret;
-}
-
 /**
  * handle_changed_spte - handle bookkeeping associated with an SPTE change
  * @kvm: kvm instance
@@ -626,6 +599,8 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
 							 struct tdp_iter *iter,
 							 u64 new_spte)
 {
+	u64 *raw_sptep = rcu_dereference(iter->sptep);
+
 	/*
 	 * The caller is responsible for ensuring the old SPTE is not a FROZEN
 	 * SPTE.  KVM should never attempt to zap or manipulate a FROZEN SPTE,
@@ -638,31 +613,46 @@ static inline int __must_check __tdp_mmu_set_spte_atomic(struct kvm *kvm,
 		int ret;
 
 		/*
-		 * Users of atomic zapping don't operate on mirror roots,
-		 * so don't handle it and bug the VM if it's seen.
+		 * KVM doesn't currently support zapping or splitting mirror
+		 * SPTEs while holding mmu_lock for read.
 		 */
-		if (KVM_BUG_ON(!is_shadow_present_pte(new_spte), kvm))
+		if (KVM_BUG_ON(is_shadow_present_pte(iter->old_spte), kvm) ||
+		    KVM_BUG_ON(!is_shadow_present_pte(new_spte), kvm))
 			return -EBUSY;
 
-		ret = set_external_spte_present(kvm, iter->sptep, iter->gfn,
-						&iter->old_spte, new_spte, iter->level);
+		/*
+		 * Temporarily freeze the SPTE until the external PTE operation
+		 * has completed, e.g. so that concurrent faults don't attempt
+		 * to install a child PTE in the external page table before the
+		 * parent PTE has been written.
+		 */
+		if (!try_cmpxchg64(raw_sptep, &iter->old_spte, FROZEN_SPTE))
+			return -EBUSY;
+
+		/*
+		 * Update the external PTE.  On success, set the mirror SPTE to
+		 * the desired value.  On failure, restore the old SPTE so that
+		 * the SPTE isn't frozen in perpetuity.
+		 */
+		ret = kvm_x86_call(set_external_spte)(kvm, iter->gfn,
+						      iter->level, new_spte);
 		if (ret)
-			return ret;
-	} else {
-		u64 *sptep = rcu_dereference(iter->sptep);
-
-		/*
-		 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs
-		 * and does not hold the mmu_lock.  On failure, i.e. if a
-		 * different logical CPU modified the SPTE, try_cmpxchg64()
-		 * updates iter->old_spte with the current value, so the caller
-		 * operates on fresh data, e.g. if it retries
-		 * tdp_mmu_set_spte_atomic()
-		 */
-		if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
-			return -EBUSY;
+			__kvm_tdp_mmu_write_spte(iter->sptep, iter->old_spte);
+		else
+			__kvm_tdp_mmu_write_spte(iter->sptep, new_spte);
+		return ret;
 	}
 
+	/*
+	 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
+	 * does not hold the mmu_lock.  On failure, i.e. if a different logical
+	 * CPU modified the SPTE, try_cmpxchg64() updates iter->old_spte with
+	 * the current value, so the caller operates on fresh data, e.g. if it
+	 * retries tdp_mmu_set_spte_atomic()
+	 */
+	if (!try_cmpxchg64(raw_sptep, &iter->old_spte, new_spte))
+		return -EBUSY;
+
 	return 0;
 }
 
-- 
2.53.0.rc1.217.geba53bf80e-goog