x86/reboot: Disable virtualization during reboot iff callback is registered

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).

Checking for a valid callback instead of simply for SVM or VMX support
can also eliminates spurious NMIs by avoiding the unecessary call to
nmi_shootdown_cpus_on_restart().

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

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 85cb2dfcb67bfb0699b415fd91376d6ca80ad68a..98e5db3fd7f42ab95eec2117b2770d26999f20db 100644 (file)
--- 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>
@@ -589,7 +588,7 @@ 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();