Re: [PATCH 2/2] x86/reboot: Disable virtualization in an emergency if SVM is supported

Thomas Gleixner
2022-05-12T10:57:15Z

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);