powerpc: ftrace, do nothing in mcount call for dyn ftrace

Impact: quicken mcount calls that are not replaced by dyn ftrace

Dynamic ftrace no longer does on the fly recording of mcount locations.
The mcount locations are now found at compile time. The mcount
function no longer needs to store registers and call a stub function.
It can now just simply return.

Since there are some functions that do not get converted to a nop
(.init sections and other code that may disappear), this patch should
help speed up that code.

Also, the stub for mcount on PowerPC 32 can not be a simple branch
link register like it is on PowerPC 64. According to the ABI specification:

"The _mcount routine is required to restore the link register from
 the stack so that the profiling code can be inserted transparently,
 whether or not the profiled function saves the link register itself."

This means that we must restore the link register that was used
to make the call to mcount.  The minimal mcount function for PPC32
ends up being:

 mcount:
        mflr    r0
        mtctr   r0
        lwz     r0, 4(r1)
        mtlr    r0
        bctr

Where we move the link register used to call mcount into the
ctr register, and then restore the link register from the stack.
Then we use the ctr register to jump back to the mcount caller.
The r0 register is free for us to use.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 9 insertions, 31 deletions

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 7ecc0d1855c3..6f7eb7e00c79 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -1162,39 +1162,17 @@ machine_check_in_rtas:
 #ifdef CONFIG_DYNAMIC_FTRACE
 _GLOBAL(mcount)
 _GLOBAL(_mcount)
-        stwu    r1,-48(r1)
-        stw     r3, 12(r1)
-        stw     r4, 16(r1)
-        stw     r5, 20(r1)
-        stw     r6, 24(r1)
-        mflr    r3
-        stw     r7, 28(r1)
-        mfcr    r5
-        stw     r8, 32(r1)
-        stw     r9, 36(r1)
-        stw     r10,40(r1)
-        stw     r3, 44(r1)
-        stw     r5, 8(r1)
-        subi    r3, r3, MCOUNT_INSN_SIZE
-        .globl mcount_call
-mcount_call:
-        bl      ftrace_stub
-        nop
-        lwz     r6, 8(r1)
-        lwz     r0, 44(r1)
-        lwz     r3, 12(r1)
+        /*
+         * It is required that _mcount on PPC32 must preserve the
+         * link register. But we have r0 to play with. We use r0
+         * to push the return address back to the caller of mcount
+         * into the ctr register, restore the link register and
+         * then jump back using the ctr register.
+         */
+        mflr    r0
         mtctr   r0
-        lwz     r4, 16(r1)
-        mtcr    r6
-        lwz     r5, 20(r1)
-        lwz     r6, 24(r1)
-        lwz     r0, 52(r1)
-        lwz     r7, 28(r1)
-        lwz     r8, 32(r1)
+        lwz     r0, 4(r1)
         mtlr    r0
-        lwz     r9, 36(r1)
-        lwz     r10,40(r1)
-        addi    r1, r1, 48
         bctr
 
 _GLOBAL(ftrace_caller)