linux-kernel - Re: [PATCH] KVM: x86: Remove outdated comments and code in kvm_on_user

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Message-ID: <20250912141132.GA85606@k08j02272.eu95sqa>
Date: Fri, 12 Sep 2025 22:11:32 +0800
From: Hou Wenlong <houwenlong.hwl@...group.com>
To: Chao Gao <chao.gao@...el.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Subject: Re: [PATCH] KVM: x86: Remove outdated comments and code in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 05:38:22PM +0800, Hou Wenlong wrote:
> On Fri, Sep 12, 2025 at 04:35:00PM +0800, Chao Gao wrote:
> > 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remove the outdated comments and local_irq_save()/local_irq_restore()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.
> >
> Thanks for pointing that out. I see that interrupts are enabled in the
> callback during the CPU offline test. I'll remove the
> lockdep_assert_irqs_disabled() here.
>

Upon a second look, can we just disable interrupts in kvm_cpu_offline()?
The other paths that call kvm_disable_virtualization_cpu() are all in an
interrupt-disabled state, although it seems that
kvm_disable_virtualization_cpu() cannot be reentered.
  
> > 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
> > >
> > >