powerpc: No need to save XER in a system call

The XER is a volatile register so there is no need to save and restore
it over a system call - zero it out in the exception stack frame
instead.

This should fix a 5 cycle stall of the mfxer/std seen on POWER7.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index e212c2f57fd89043d761f9f14245a414c066d70f..7f1e6de9a5a9e6d68188d7b14ff877278dc45cb5 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -82,6 +82,7 @@ system_call_common:
        std     r11,GPR10(r1)
        std     r11,GPR11(r1)
        std     r11,GPR12(r1)
+       std     r11,_XER(r1)
        std     r9,GPR13(r1)
        mfcr    r9
        mflr    r10
@@ -89,9 +90,7 @@ system_call_common:
        std     r9,_CCR(r1)
        std     r10,_LINK(r1)
        std     r11,_TRAP(r1)
-       mfxer   r9
        mfctr   r10
-       std     r9,_XER(r1)
        std     r10,_CTR(r1)
        std     r3,ORIG_GPR3(r1)
        ld      r2,PACATOC(r13)