powerpc64, ftrace: save toc only on modules for function graph

The TOCS used by modules are different than the one used by
the core kernel code. The function graph tracer must save and
restore the TOC whenever it traces a module call. But this
is an added overhead to burden the majority of core kernel
code being traced.

Benjamin Herrenschmidt suggested in testing the entry of
the call to tell if it is a core kernel function or a module.
He recommended using the REGION_ID() macro to perform this test.

This patch implements Benjamin's idea, and uses a different
return_to_handler routine dependent on if the entry is a core
kernel function or not. The module version saves the TOC, where as
the core kernel version does not.

Geoff Lavand tested on PS3.

Tested-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 26 insertions, 1 deletions

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 9f61fd61f277..abfc32330479 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -971,6 +971,28 @@ _GLOBAL(ftrace_graph_caller)
 
 _GLOBAL(return_to_handler)
         /* need to save return values */
+        std     r4,  -24(r1)
+        std     r3,  -16(r1)
+        std     r31, -8(r1)
+        mr      r31, r1
+        stdu    r1, -112(r1)
+
+        bl      .ftrace_return_to_handler
+        nop
+
+        /* return value has real return address */
+        mtlr    r3
+
+        ld      r1, 0(r1)
+        ld      r4,  -24(r1)
+        ld      r3,  -16(r1)
+        ld      r31, -8(r1)
+
+        /* Jump back to real return address */
+        blr
+
+_GLOBAL(mod_return_to_handler)
+        /* need to save return values */
         std     r4,  -32(r1)
         std     r3,  -24(r1)
         /* save TOC */
@@ -979,7 +1001,10 @@ _GLOBAL(return_to_handler)
         mr      r31, r1
         stdu    r1, -112(r1)
 
-        /* update the TOC */
+        /*
+         * We are in a module using the module's TOC.
+         * Switch to our TOC to run inside the core kernel.
+         */
         LOAD_REG_IMMEDIATE(r4,ftrace_return_to_handler)
         ld      r2, 8(r4)