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

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

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