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

[Christophe Leroy]

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 at 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.

Christophe