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

[Alan Modra <amodra@gmail.com>]

On Wed, Jan 27, 2016 at 09:19:27PM +1100, Michael Ellerman wrote:
> Hi Torsten,
> 
> On Mon, 2016-01-25 at 16:26 +0100, Torsten Duwe wrote:
> > 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 <duwe@suse.de>
> > ---
> >  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.

r12 might be a better choice for a temp reg.

-- 
Alan Modra
Australia Development Lab, IBM