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>

arch/x86/include/asm/virtext.h

@@ -70,16 +70,6 @@ static inline void __cpu_emergency_vmxoff(void)
 		cpu_vmxoff();
 }
 
-/** Disable VMX if it is supported and enabled on the current CPU
- */
-static inline void cpu_emergency_vmxoff(void)
-{
-	if (cpu_has_vmx())
-		__cpu_emergency_vmxoff();
-}
-
-
-
 
 /*
  * SVM functions:

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();
 }
 

arch/x86/kvm/vmx/vmx.c

@@ -725,14 +725,16 @@ static int vmx_set_guest_uret_msr(struct vcpu_vmx *vmx,
 	return ret;
 }
 
-static void crash_vmclear_local_loaded_vmcss(void)
+static void vmx_emergency_disable(void)
 {
 	int cpu = raw_smp_processor_id();
 	struct loaded_vmcs *v;
 
 	list_for_each_entry(v, &per_cpu(loaded_vmcss_on_cpu, cpu),
 			    loaded_vmcss_on_cpu_link)
 		vmcs_clear(v->vmcs);
+
+	__cpu_emergency_vmxoff();
 }
 
 static void __loaded_vmcs_clear(void *arg)
@@ -8571,7 +8573,7 @@ static void __vmx_exit(void)
 {
 	allow_smaller_maxphyaddr = false;
 
-	cpu_emergency_unregister_virt_callback(crash_vmclear_local_loaded_vmcss);
+	cpu_emergency_unregister_virt_callback(vmx_emergency_disable);
 
 	vmx_cleanup_l1d_flush();
 }
@@ -8619,7 +8621,7 @@ static int __init vmx_init(void)
 		pi_init_cpu(cpu);
 	}
 
-	cpu_emergency_register_virt_callback(crash_vmclear_local_loaded_vmcss);
+	cpu_emergency_register_virt_callback(vmx_emergency_disable);
 
 	vmx_check_vmcs12_offsets();