linux-kernel - [PATCH v3 2/2] KVM: x86: Change the outdated comments and code in kvm_on_user

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Message-Id: <4d65465545b2ccebda3a0c3e601320578024ed3b.1758002303.git.houwenlong.hwl@antgroup.com>
Date: Tue, 16 Sep 2025 14:07:36 +0800
From: Hou Wenlong <houwenlong.hwl@...group.com>
To: kvm@...r.kernel.org
Cc: Chao Gao <chao.gao@...el.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] KVM: x86: Change the outdated comments and code in kvm_on_user_return()

The commit a377ac1cd9d7b ("x86/entry: Move user return notifier out of
loop") moved fire_user_return_notifiers() into the section with IRQs
disabled, and it somewhat inadvertantly fixed the underlying issue that
was papered over by commit 1650b4ebc99d ("KVM: Disable irq while
unregistering user notifier").  Therefore, the comments and code are
outdated.  Aslo assert that IRQs are disabled in kvm_on_user_return(),
as both fire_user_return_notifiers() and
kvm_arch_disable_virtualization_cpu() are now in IRQs disabled state.

Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
---
 arch/x86/kvm/x86.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 33fba801b205..84fc30a99be1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -568,18 +568,18 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
 	struct kvm_user_return_msrs *msrs
 		= container_of(urn, struct kvm_user_return_msrs, urn);
 	struct kvm_user_return_msr_values *values;
-	unsigned long flags;
 
 	/*
-	 * Disabling irqs at this point since the following code could be
-	 * interrupted and executed through kvm_arch_disable_virtualization_cpu()
+	 * Assert that IRQs are disabled.  KVM disables virtualization via IPI
+	 * callback on reboot, and this code isn't safe for re-entrancy, e.g.
+	 * receiving the IRQ after checking "registered" would lead to double
+	 * deletion of KVM's notifier.
 	 */
-	local_irq_save(flags);
-	if (msrs->registered) {
-		msrs->registered = false;
-		user_return_notifier_unregister(urn);
-	}
-	local_irq_restore(flags);
+	lockdep_assert_irqs_disabled();
+
+	msrs->registered = false;
+	user_return_notifier_unregister(urn);
+
 	for (slot = 0; slot < kvm_nr_uret_msrs; ++slot) {
 		values = &msrs->values[slot];
 		if (values->host != values->curr) {
-- 
2.31.1