x86/reboot: Expose VMCS crash hooks if and only if KVM_{INTEL,AMD} is enabled

Expose the crash/reboot hooks used by KVM to disable virtualization in
hardware and unblock INIT only if there's a potential in-tree user,
i.e. either KVM_INTEL or KVM_AMD is enabled.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/reboot.h

@@ -25,10 +25,14 @@ void __noreturn machine_real_restart(unsigned int type);
 #define MRR_BIOS	0
 #define MRR_APM		1
 
+#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
 typedef void (cpu_emergency_virt_cb)(void);
 void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
 void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
 void cpu_emergency_disable_virtualization(void);
+#else
+static inline void cpu_emergency_disable_virtualization(void) {}
+#endif /* CONFIG_KVM_INTEL || CONFIG_KVM_AMD */
 
 typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
 void nmi_shootdown_cpus(nmi_shootdown_cb callback);

arch/x86/kernel/reboot.c

@@ -529,6 +529,7 @@ static inline void kb_wait(void)
 
 static inline void nmi_shootdown_cpus_on_restart(void);
 
+#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
 /* RCU-protected callback to disable virtualization prior to reboot. */
 static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
 
@@ -596,7 +597,9 @@ 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)
 {