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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sean Christopherson <seanjc@google.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Cc:     "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        "Guilherme G . Piccoli" <gpiccoli@igalia.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Subject: Re: [PATCH 2/2] x86/reboot: Disable virtualization in an emergency
 if SVM is supported
In-Reply-To: <20220511234332.3654455-3-seanjc@google.com>
References: <20220511234332.3654455-1-seanjc@google.com>
 <20220511234332.3654455-3-seanjc@google.com>
Date:   Thu, 12 May 2022 12:57:06 +0200
Message-ID: <87tu9vvx19.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, May 11 2022 at 23:43, Sean Christopherson wrote:
> Disable SVM on all CPUs via NMI shootdown during an emergency reboot.
> Like VMX, SVM can block INIT and thus prevent bringing up other CPUs via
> INIT-SIPI-SIPI.

With the delta patch applied, I'd make that:

--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -530,29 +530,25 @@ static inline void kb_wait(void)
 
 static inline void nmi_shootdown_cpus_on_restart(void);
 
-/* Use NMIs as IPIs to tell all CPUs to disable virtualization */
-static void emergency_vmx_disable_all(void)
+static void emergency_reboot_disable_virtualization(void)
 {
 	/* Just make sure we won't change CPUs while doing this */
 	local_irq_disable();
 
 	/*
-	 * Disable VMX on all CPUs before rebooting, otherwise we risk hanging
-	 * the machine, because the CPU blocks INIT when it's in VMX root.
+	 * Disable virtualization on all CPUs before rebooting to avoid hanging
+	 * the system, as VMX and SVM block INIT when running in the host
 	 *
 	 * 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 exit VMX root and halt.
+	 * use NMIs as IPIs to tell the other CPUs to disable VMX/SVM and halt.
 	 *
-	 * Do the NMI shootdown even if VMX if off on _this_ CPU, as that
-	 * doesn't prevent a different CPU from being in VMX root operation.
+	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
+	 * other CPUs may have virtualization enabled.
 	 */
-	if (cpu_has_vmx()) {
-		/* Safely force _this_ CPU out of VMX root operation. */
-		__cpu_emergency_vmxoff();
+	cpu_crash_disable_virtualization();
 
-		/* Halt and exit VMX root operation on the other CPUs. */
+	if (cpu_has_vmx() || cpu_has_svm(NULL))
 		nmi_shootdown_cpus_on_restart();
-	}
 }
 
 
@@ -587,7 +583,7 @@ static void native_machine_emergency_res
 	unsigned short mode;
 
 	if (reboot_emergency)
-		emergency_vmx_disable_all();
+		emergency_reboot_disable_virtualization();
 
 	tboot_shutdown(TB_SHUTDOWN_REBOOT);