x86, function-graph: only save return values on x86_64

Impact: speed up

The return to handler portion of the function graph tracer should only
need to save the return values. The caller already saved off the
registers that the callee can modify. The returning function already
saved the registers it modified. When we call our own trace function
it too will save the registers that the callee must restore.

There's no reason to save off anything more that the registers used
to return the values.

Note, I did a complete kernel build with this modification and the
function graph tracer running on x86_64.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -147,27 +147,14 @@ END(ftrace_graph_caller)
 GLOBAL(return_to_handler)
 	subq  $80, %rsp
 
+	/* Save the return values */
 	movq %rax, (%rsp)
-	movq %rcx, 8(%rsp)
-	movq %rdx, 16(%rsp)
-	movq %rsi, 24(%rsp)
-	movq %rdi, 32(%rsp)
-	movq %r8, 40(%rsp)
-	movq %r9, 48(%rsp)
-	movq %r10, 56(%rsp)
-	movq %r11, 64(%rsp)
+	movq %rdx, 8(%rsp)
 
 	call ftrace_return_to_handler
 
 	movq %rax, 72(%rsp)
-	movq 64(%rsp), %r11
-	movq 56(%rsp), %r10
-	movq 48(%rsp), %r9
-	movq 40(%rsp), %r8
-	movq 32(%rsp), %rdi
-	movq 24(%rsp), %rsi
-	movq 16(%rsp), %rdx
-	movq 8(%rsp), %rcx
+	movq 8(%rsp), %rdx
 	movq (%rsp), %rax
 	addq $72, %rsp
 	retq