linux-kernel - [PATCH v2 5/5] KVM: x86/mmu: Check and free obsolete roots in kvm_mmu

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Message-ID: <20250318013333.5817-1-yan.y.zhao@intel.com>
Date: Tue, 18 Mar 2025 09:33:33 +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,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: [PATCH v2 5/5] KVM: x86/mmu: Check and free obsolete roots in kvm_mmu_reload()

Check request KVM_REQ_MMU_FREE_OBSOLETE_ROOTS to free obsolete roots in
kvm_mmu_reload() to prevent kvm_mmu_reload() from seeing a stale obsolete
root.

Since kvm_mmu_reload() can be called outside the
vcpu_enter_guest() path (e.g., kvm_arch_vcpu_pre_fault_memory()), it may be
invoked after a root has been marked obsolete and before vcpu_enter_guest()
is invoked to process KVM_REQ_MMU_FREE_OBSOLETE_ROOTS and set root.hpa to
invalid. This causes kvm_mmu_reload() to fail to load a new root, which
can lead to kvm_arch_vcpu_pre_fault_memory() being stuck in the while
loop in kvm_tdp_map_page() since RET_PF_RETRY is always returned due to
is_page_fault_stale().

Keep the existing check of KVM_REQ_MMU_FREE_OBSOLETE_ROOTS in
vcpu_enter_guest() since the cost of kvm_check_request() is negligible,
especially a check that's guarded by kvm_request_pending().

Export symbol of kvm_mmu_free_obsolete_roots() as kvm_mmu_reload() is
inline and may be called outside of kvm.ko.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
 arch/x86/kvm/mmu.h     | 3 +++
 arch/x86/kvm/mmu/mmu.c | 1 +
 2 files changed, 4 insertions(+)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 050a0e229a4d..f2b36d32ef40 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -104,6 +104,9 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,

 static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
 {
+	if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
+		kvm_mmu_free_obsolete_roots(vcpu);
+
 	/*
 	 * Checking root.hpa is sufficient even when KVM has mirror root.
 	 * We can have either:
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 607cbb19ea96..15fd4838e4f2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5802,6 +5802,7 @@ void kvm_mmu_free_obsolete_roots(struct kvm_vcpu *vcpu)
 	__kvm_mmu_free_obsolete_roots(vcpu->kvm, &vcpu->arch.root_mmu);
 	__kvm_mmu_free_obsolete_roots(vcpu->kvm, &vcpu->arch.guest_mmu);
 }
+EXPORT_SYMBOL_GPL(kvm_mmu_free_obsolete_roots);

 static u64 mmu_pte_write_fetch_gpte(struct kvm_vcpu *vcpu, gpa_t *gpa,
 				    int *bytes)
-- 
2.43.2