x86/reboot: KVM: Handle VMXOFF in KVM's reboot callback

Use KVM VMX's reboot/crash callback to do VMXOFF in an emergency instead
of manually and blindly doing VMXOFF.  There's no need to attempt VMXOFF
if a hypervisor, i.e. KVM, isn't loaded/active, i.e. if the CPU can't
possibly be post-VMXON.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 9 insertions, 20 deletions

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 62ccedeb5e2b..d2d0f2672a64 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -787,13 +787,7 @@ void machine_crash_shutdown(struct pt_regs *regs)
 }
 #endif
 
-/*
- * This is used to VMCLEAR all VMCSs loaded on the
- * processor. And when loading kvm_intel module, the
- * callback function pointer will be assigned.
- *
- * protected by rcu.
- */
+/* RCU-protected callback to disable virtualization prior to reboot. */
 static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
 
 void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback)
@@ -815,17 +809,6 @@ void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback)
 }
 EXPORT_SYMBOL_GPL(cpu_emergency_unregister_virt_callback);
 
-static inline void cpu_crash_vmclear_loaded_vmcss(void)
-{
-	cpu_emergency_virt_cb *callback;
-
-	rcu_read_lock();
-	callback = rcu_dereference(cpu_emergency_virt_callback);
-	if (callback)
-		callback();
-	rcu_read_unlock();
-}
-
 /* This is the CPU performing the emergency shutdown work. */
 int crashing_cpu = -1;
 
@@ -836,9 +819,15 @@ int crashing_cpu = -1;
  */
 void cpu_emergency_disable_virtualization(void)
 {
-	cpu_crash_vmclear_loaded_vmcss();
+	cpu_emergency_virt_cb *callback;
+
+	rcu_read_lock();
+	callback = rcu_dereference(cpu_emergency_virt_callback);
+	if (callback)
+		callback();
+	rcu_read_unlock();
 
-	cpu_emergency_vmxoff();
+	/* KVM_AMD doesn't yet utilize the common callback. */
 	cpu_emergency_svm_disable();
 }