powerpc: Reintroduce is_kvm_guest() as a fast-path check

Introduce a static branch that would be set during boot if the OS
happens to be a KVM guest. Subsequent checks to see if we are on KVM
will rely on this static branch. This static branch would be used in
vcpu_is_preempted() in a subsequent patch.

Signed-off-by: Srikar Dronamraju <[e-mail]>
Acked-by: Waiman Long <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/include/asm/kvm_guest.h

···
 #define _ASM_POWERPC_KVM_GUEST_H_
 
 #if defined(CONFIG_PPC_PSERIES) || defined(CONFIG_KVM_GUEST)
+#include <linux/jump_label.h>
+
+DECLARE_STATIC_KEY_FALSE(kvm_guest);
+
+static inline bool is_kvm_guest(void)
+{
+	return static_branch_unlikely(&kvm_guest);
+}
+
 bool check_kvm_guest(void);
 #else
+static inline bool is_kvm_guest(void) { return false; }
 static inline bool check_kvm_guest(void) { return false; }
 #endif
 

arch/powerpc/include/asm/kvm_para.h

···
 
 static inline int kvm_para_available(void)
 {
-	return IS_ENABLED(CONFIG_KVM_GUEST) && check_kvm_guest();
+	return IS_ENABLED(CONFIG_KVM_GUEST) && is_kvm_guest();
 }
 
 static inline unsigned int kvm_arch_para_features(void)

arch/powerpc/kernel/firmware.c

···
 #endif
 
 #if defined(CONFIG_PPC_PSERIES) || defined(CONFIG_KVM_GUEST)
+DEFINE_STATIC_KEY_FALSE(kvm_guest);
 bool check_kvm_guest(void)
 {
 	struct device_node *hyper_node;
···
 	if (!of_device_is_compatible(hyper_node, "linux,kvm"))
 		return false;
 
+	static_branch_enable(&kvm_guest);
 	return true;
 }
 #endif