linux-kernel - Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement to kernel

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Message-ID: <aTe4QyE3h8LHOAMb@intel.com>
Date: Tue, 9 Dec 2025 13:48:51 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: 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>, Kiryl Shutsemau <kas@...nel.org>, Paolo Bonzini
	<pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>,
	<linux-coco@...ts.linux.dev>, <kvm@...r.kernel.org>, Dan Williams
	<dan.j.williams@...el.com>
Subject: Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement
 to kernel

>--- /dev/null
>+++ b/arch/x86/include/asm/virt.h
>@@ -0,0 +1,26 @@
>+/* SPDX-License-Identifier: GPL-2.0-only */
>+#ifndef _ASM_X86_VIRT_H
>+#define _ASM_X86_VIRT_H
>+
>+#include <asm/reboot.h>

asm/reboot.h isn't used.

>+
>+typedef void (cpu_emergency_virt_cb)(void);
>+
>+#if IS_ENABLED(CONFIG_KVM_X86)
>+extern bool virt_rebooting;
>+
>+void __init x86_virt_init(void);
>+
>+int x86_virt_get_cpu(int feat);
>+void x86_virt_put_cpu(int feat);
>+
>+int x86_virt_emergency_disable_virtualization_cpu(void);
>+
>+void x86_virt_register_emergency_callback(cpu_emergency_virt_cb *callback);
>+void x86_virt_unregister_emergency_callback(cpu_emergency_virt_cb *callback);
>+#else
>+static __always_inline void x86_virt_init(void) {}

Why does this need to be "__always_inline" rather than just "inline"?

> static void emergency_reboot_disable_virtualization(void)
> {
> 	local_irq_disable();
>@@ -587,16 +543,11 @@ static void emergency_reboot_disable_virtualization(void)
> 	 * We can't take any locks and we may be on an inconsistent state, so
> 	 * use NMIs as IPIs to tell the other CPUs to disable VMX/SVM and halt.
> 	 *
>-	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
>-	 * other CPUs may have virtualization enabled.
>+	 * Safely force _this_ CPU out of VMX/SVM operation, and if necessary,
>+	 * blast NMIs to force other CPUs out of VMX/SVM as well.k

								 ^ stray "k".

I don't understand the "if necessary" part. My understanding is this code
issues NMIs if CPUs support VMX or SVM. If so, I think the code snippet below
would be more readable:

	if (cpu_feature_enabled(X86_FEATURE_VMX) ||
	    cpu_feature_enabled(X86_FEATURE_SVM)) {
		x86_virt_emergency_disable_virtualization_cpu();
		nmi_shootdown_cpus_on_restart();
	}

Then x86_virt_emergency_disable_virtualization_cpu() wouldn't need to return
anything. And readers wouldn't need to trace down the function to understand
when NMIs are "necessary" and when they are not.

> 	 */
>-	if (rcu_access_pointer(cpu_emergency_virt_callback)) {
>-		/* Safely force _this_ CPU out of VMX/SVM operation. */
>-		cpu_emergency_disable_virtualization();
>-
>-		/* Disable VMX/SVM and halt on other CPUs. */
>+	if (!x86_virt_emergency_disable_virtualization_cpu())
> 		nmi_shootdown_cpus_on_restart();
>-	}
> }

<snip>

>+#define x86_virt_call(fn)				\
>+({							\
>+	int __r;					\
>+							\
>+	if (IS_ENABLED(CONFIG_KVM_INTEL) &&		\
>+	    cpu_feature_enabled(X86_FEATURE_VMX))	\
>+		__r = x86_vmx_##fn();			\
>+	else if (IS_ENABLED(CONFIG_KVM_AMD) &&		\
>+		 cpu_feature_enabled(X86_FEATURE_SVM))	\
>+		__r = x86_svm_##fn();			\
>+	else						\
>+		__r = -EOPNOTSUPP;			\
>+							\
>+	__r;						\
>+})
>+
>+int x86_virt_get_cpu(int feat)
>+{
>+	int r;
>+
>+	if (!x86_virt_feature || x86_virt_feature != feat)
>+		return -EOPNOTSUPP;
>+
>+	if (this_cpu_inc_return(virtualization_nr_users) > 1)
>+		return 0;

Should we assert that preemption is disabled? Calling this API when preemption
is enabled is wrong.

Maybe use __this_cpu_inc_return(), which already verifies preemption status.

<snip>

>+int x86_virt_emergency_disable_virtualization_cpu(void)
>+{
>+	if (!x86_virt_feature)
>+		return -EOPNOTSUPP;
>+
>+	/*
>+	 * IRQs must be disabled as virtualization is enabled in hardware via
>+	 * function call IPIs, i.e. IRQs need to be disabled to guarantee
>+	 * virtualization stays disabled.
>+	 */
>+	lockdep_assert_irqs_disabled();
>+
>+	/*
>+	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
>+	 * other CPUs may have virtualization enabled.
>+	 *
>+	 * TODO: Track whether or not virtualization might be enabled on other
>+	 *	 CPUs?  May not be worth avoiding the NMI shootdown...
>+	 */

This comment is misplaced. NMIs are issued by the caller.

>+	(void)x86_virt_call(emergency_disable_virtualization_cpu);
>+	return 0;
>+}