KVM: arm64: selftests: Add support for cpu_relax

Implement the guest helper routine, cpu_relax(), to yield
the processor to other tasks.

The function was derived from
arch/arm64/include/asm/vdso/processor.h.

Signed-off-by: Raghavendra Rao Ananta <[email protected]>
Reviewed-by: Oliver Upton <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: rananta468

tools/testing/selftests/kvm/include/aarch64/processor.h

@@ -122,6 +122,11 @@ void vm_install_exception_handler(struct kvm_vm *vm,
 void vm_install_sync_handler(struct kvm_vm *vm,
 		int vector, int ec, handler_fn handler);
 
+static inline void cpu_relax(void)
+{
+	asm volatile("yield" ::: "memory");
+}
+
 #define isb()		asm volatile("isb" : : : "memory")
 #define dsb(opt)	asm volatile("dsb " #opt : : : "memory")
 #define dmb(opt)	asm volatile("dmb " #opt : : : "memory")