linux-kernel - Re: [PATCH] KVM: x86: Remove outdated comments and code in kvm_on_user

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Message-ID: <aMPbNBofTCFGTCs6@intel.com>
Date: Fri, 12 Sep 2025 16:35:00 +0800
From: Chao Gao <chao.gao@...el.com>
To: Hou Wenlong <houwenlong.hwl@...group.com>
CC: <kvm@...r.kernel.org>, 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Subject: Re: [PATCH] KVM: x86: Remove outdated comments and code in
 kvm_on_user_return()

On Fri, Sep 12, 2025 at 03:35:29PM +0800, Hou Wenlong wrote:
>The commit a377ac1cd9d7b ("x86/entry: Move user return notifier out of
>loop") moved fire_user_return_notifiers() into the section with
>interrupts disabled, so the callback kvm_on_user_return() cannot be
>interrupted by kvm_arch_disable_virtualization_cpu() now. Therefore,
>remove the outdated comments and local_irq_save()/local_irq_restore()
>code in kvm_on_user_return().
>
>Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
>---
> arch/x86/kvm/x86.c | 16 +++++-----------
> 1 file changed, 5 insertions(+), 11 deletions(-)
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 33fba801b205..10afbacb1851 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -568,18 +568,12 @@ 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()
>-	 */
>-	local_irq_save(flags);
>-	if (msrs->registered) {
>-		msrs->registered = false;
>-		user_return_notifier_unregister(urn);
>-	}
>-	local_irq_restore(flags);
>+	lockdep_assert_irqs_disabled();

kvm_offline_cpu() may call into this function. But I am not sure if interrupts
are disabled in that path.

Documentation/core-api/cpu_hotplug.rst says that callbacks in the ONLINE section
are invoked with interrupts and preemption enabled.

>+
>+	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
>
>