KVM: selftests: Define cpu_relax() helpers for s390 and x86

Add cpu_relax() for s390 and x86 for use in arch-agnostic tests.  arm64
already defines its own version.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-28-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/include/s390x/processor.h

@@ -5,6 +5,8 @@
 #ifndef SELFTEST_KVM_PROCESSOR_H
 #define SELFTEST_KVM_PROCESSOR_H
 
+#include <linux/compiler.h>
+
 /* Bits in the region/segment table entry */
 #define REGION_ENTRY_ORIGIN	~0xfffUL /* region/segment table origin	   */
 #define REGION_ENTRY_PROTECT	0x200	 /* region protection bit	   */
@@ -19,4 +21,10 @@
 #define PAGE_PROTECT	0x200		/* HW read-only bit  */
 #define PAGE_NOEXEC	0x100		/* HW no-execute bit */
 
+/* Is there a portable way to do this? */
+static inline void cpu_relax(void)
+{
+	barrier();
+}
+
 #endif

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -363,6 +363,11 @@ static inline unsigned long get_xmm(int n)
 	return 0;
 }
 
+static inline void cpu_relax(void)
+{
+	asm volatile("rep; nop" ::: "memory");
+}
+
 bool is_intel_cpu(void);
 bool is_amd_cpu(void);