Re: [PATCH 1/5] powerpc/64: use 32-bit immediate for STACK_FRAME_REGS_MARKER

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Sep 23, 2022 at 3:34 PM AEST, Christophe Leroy wrote:
>
>
> Le 23/09/2022 à 05:25, Nicholas Piggin a écrit :
> > Using a 32-bit constant for this marker allows it to be loaded with
> > two ALU instructions, like 32-bit. This avoids a TOC entry and a
> > TOC load that depends on the r2 value that has just been loaded from
> > the PACA.
> > 
> > This changes the value for 32-bit as well, so both have the same
> > value in the low 4 bytes and 64-bit has 0xffffffff in the top bytes.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> >   arch/powerpc/include/asm/ptrace.h    | 5 +++--
> >   arch/powerpc/kernel/entry_32.S       | 6 +++---
> >   arch/powerpc/kernel/exceptions-64e.S | 8 +-------
> >   arch/powerpc/kernel/exceptions-64s.S | 2 +-
> >   arch/powerpc/kernel/head_64.S        | 7 -------
> >   arch/powerpc/kernel/interrupt_64.S   | 6 +++---
> >   6 files changed, 11 insertions(+), 23 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> > index a03403695cd4..49d720bb888b 100644
> > --- a/arch/powerpc/include/asm/ptrace.h
> > +++ b/arch/powerpc/include/asm/ptrace.h
> > @@ -99,6 +99,9 @@ struct pt_regs
> >   
> >   #define STACK_FRAME_WITH_PT_REGS (STACK_FRAME_OVERHEAD + sizeof(struct pt_regs))
> >   
> > +/* 0xffffffff8d9a988d on 64-bit */
> > +#define STACK_FRAME_REGS_MARKER	ASM_CONST(-0x72656773) /* 0x8d9a988d */
> > +
>
> 0x72656773 is "REGS" in ASCII (Big Endian) and you can spot it 
> immediatly in a memory dump.
> 0x7265677368657265 is "REGSHERE".
>
> 0x8d9a988d is not printable.
>
> Don't know if it can be a problem.

Oh. I guess it doesn't really matter if it has zeroes or f in the
top 4 bytes so I should keep it "REGS" then. I was thinking about
later moving it into the reserved word next to the CR word on
64-bit so we would only have 4 bytes for it anyway.

Thanks,
Nick