[PATCH 08/16] x86/reboot: Disable virtualization during reboot iff callback is registered

[Sean Christopherson <seanjc@google.com>]

Attempt to disable virtualization during an emergency reboot if and only
if there is a registered virt callback, i.e. iff a hypervisor (KVM) is
active.  If there's no active hypervisor, then the CPU can't be operating
with VMX or SVM enabled (barring an egregious bug).

Note, IRQs are disabled, which prevents KVM from coming along and enabling
virtualization after the fact.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/reboot.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index a006ddaa1405..e0dd1b00ba01 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -22,7 +22,6 @@
 #include <asm/reboot_fixups.h>
 #include <asm/reboot.h>
 #include <asm/pci_x86.h>
-#include <asm/virtext.h>
 #include <asm/cpu.h>
 #include <asm/nmi.h>
 #include <asm/smp.h>
@@ -570,7 +569,6 @@ void cpu_emergency_disable_virtualization(void)
 		callback();
 	rcu_read_unlock();
 }
-#endif /* CONFIG_KVM_INTEL || CONFIG_KVM_AMD */
 
 static void emergency_reboot_disable_virtualization(void)
 {
@@ -587,7 +585,7 @@ static void emergency_reboot_disable_virtualization(void)
 	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
 	 * other CPUs may have virtualization enabled.
 	 */
-	if (cpu_has_vmx() || cpu_has_svm(NULL)) {
+	if (rcu_access_pointer(cpu_emergency_virt_callback)) {
 		/* Safely force _this_ CPU out of VMX/SVM operation. */
 		cpu_emergency_disable_virtualization();
 
@@ -595,6 +593,9 @@ static void emergency_reboot_disable_virtualization(void)
 		nmi_shootdown_cpus_on_restart();
 	}
 }
+#else
+static void emergency_reboot_disable_virtualization(void) { }
+#endif /* CONFIG_KVM_INTEL || CONFIG_KVM_AMD */
 
 
 void __attribute__((weak)) mach_reboot_fixups(void)
-- 
2.39.0.rc0.267.gcb52ba06e7-goog