Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

On Wed, 2016-01-27 at 23:28 +1030, Alan Modra wrote:

On Wed, Jan 27, 2016 at 09:19:27PM +1100, Michael Ellerman wrote:

quoted

Hi Torsten,

On Mon, 2016-01-25 at 16:26 +0100, Torsten Duwe wrote:

quoted

The gcc switch -mprofile-kernel, available for ppc64 on gcc > 4.8.5,
allows to call _mcount very early in the function, which low-level
ASM code and code patching functions need to consider.
Especially the link register and the parameter registers are still
alive and not yet saved into a new stack frame.

Signed-off-by: Torsten Duwe <redacted>
---
 arch/powerpc/kernel/entry_64.S  | 45 +++++++++++++++++++++++++++++++++++++++--
 arch/powerpc/kernel/ftrace.c    | 12 +++++++++--
 arch/powerpc/kernel/module_64.c | 14 +++++++++++++
 3 files changed, 67 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index a94f155..e7cd043 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -1206,7 +1206,12 @@ _GLOBAL(enter_prom)
 #ifdef CONFIG_DYNAMIC_FTRACE
 _GLOBAL(mcount)
 _GLOBAL(_mcount)
-	blr
+	std	r0,LRSAVE(r1) /* gcc6 does this _after_ this call _only_ */
+	mflr	r0
+	mtctr	r0
+	ld	r0,LRSAVE(r1)
+	mtlr	r0
+	bctr

Can we use r11 instead? eg:

_GLOBAL(_mcount)
	mflr	r11
	mtctr	r11
	mtlr	r0
	bctr

Depends on what you need to support.  As Torsten says, the code to
call _mcount when -mprofile-kernel is emitted before the prologue of a
function (similar to -m32), but after the ELFv2 global entry point
code.  If you trash r11 here you're killing the static chain pointer,
used by C nested functions or other languages that use a static chain,
eg. Pascal.  r11 has *not* been saved for ELFv2.

OK, thanks for clarfiying. Pascal is not a big concern :D. But although I
don't think we use nested functions anywhere, someone somewhere could be, or at
least might one day.

r12 might be a better choice for a temp reg.

Even better.

cheers