[RFC PATCH] arm64: Implement cpu_relax as yield

[Peter Crosthwaite]

ARM64 has the yield nop hint which has the intended semantics of
cpu_relax. Implement.

The immediate application is ARM CPU emulators. An emulator can take
advantage of the yield hint to de-prioritise an emulated CPU in favor
of other emulation tasks. QEMU A64 SMP emulation has yield awareness,
and sees a significant boot time performance increase with this change.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite at xilinx.com>
---
 arch/arm64/include/asm/processor.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index f9be30e..ac2381d 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -126,7 +126,11 @@ extern void release_thread(struct task_struct *);
 
 unsigned long get_wchan(struct task_struct *p);
 
-#define cpu_relax()			barrier()
+static inline void cpu_relax(void)
+{
+	asm volatile("yield" ::: "memory");
+}
+
 #define cpu_relax_lowlatency()                cpu_relax()
 
 /* Thread switching */
-- 
2.3.0.1.g27a12f1