Re: [PATCH 2/4] alpha: stop storing the user stack pointer in pt_regs

[Magnus Lindholm]

Hi Matt,


On Sun, Oct 11, 2026 at 10:16 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> Nothing reads pt_regs->usp now that user_stack_pointer() uses the PCB.
> Drop the stores, which saves a PAL_rdusp call on every syscall, and
> give the field back its old name.
>
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
> ---
>  arch/alpha/include/uapi/asm/ptrace.h | 2 +-
>  arch/alpha/kernel/asm-offsets.c      | 1 -
>  arch/alpha/kernel/entry.S            | 5 -----
>  arch/alpha/kernel/traps.c            | 8 --------
>  4 files changed, 1 insertion(+), 15 deletions(-)
>
> diff --git a/arch/alpha/include/uapi/asm/ptrace.h b/arch/alpha/include/uapi/asm/ptrace.h
> index 9d86b2a1526e..72ed913a910f 100644
> --- a/arch/alpha/include/uapi/asm/ptrace.h
> +++ b/arch/alpha/include/uapi/asm/ptrace.h
> @@ -43,7 +43,7 @@ struct pt_regs {
>         unsigned long trap_a1;
>         unsigned long trap_a2;
>  /* This makes the stack 16-byte aligned as GCC expects */
> -       unsigned long usp;
> +       unsigned long __pad0;
>  /* These are saved by PAL-code: */
>         unsigned long ps;
>         unsigned long pc;
> diff --git a/arch/alpha/kernel/asm-offsets.c b/arch/alpha/kernel/asm-offsets.c
> index c89792c86044..22ed3c975b1c 100644
> --- a/arch/alpha/kernel/asm-offsets.c
> +++ b/arch/alpha/kernel/asm-offsets.c
> @@ -30,7 +30,6 @@ static void __used foo(void)
>
>         DEFINE(HAE_CACHE, offsetof(struct alpha_machine_vector, hae_cache));
>         DEFINE(HAE_REG, offsetof(struct alpha_machine_vector, hae_register));
> -       DEFINE(PT_REGS_USP, offsetof(struct pt_regs, usp));
>
>         DEFINE(TI_FLAGS,        offsetof(struct thread_info, flags));
>         DEFINE(TI_SYSCALL_WORK, offsetof(struct thread_info, syscall_work));
> diff --git a/arch/alpha/kernel/entry.S b/arch/alpha/kernel/entry.S
> index bfa116d455fb..790389a15c3c 100644
> --- a/arch/alpha/kernel/entry.S
> +++ b/arch/alpha/kernel/entry.S
> @@ -476,11 +476,6 @@ entSys:
>         ldq     $1, 0($sp)      /* syscall nr from saved r0 */
>         stq     $1, 8($sp)      /* regs->r1 = shadow syscall nr */
>
> -       mov     $0, $8
> -       call_pal PAL_rdusp
> -       stq     $0, PT_REGS_USP($sp)
> -       mov     $8, $0
> -
>         lda     $8, 0x3fff
>         bic     $sp, $8, $8
>
> diff --git a/arch/alpha/kernel/traps.c b/arch/alpha/kernel/traps.c
> index cfaef96c4ce8..5882c65b7ef1 100644
> --- a/arch/alpha/kernel/traps.c
> +++ b/arch/alpha/kernel/traps.c
> @@ -30,12 +30,6 @@
>
>  #include "proto.h"
>
> -static __always_inline void alpha_snapshot_usp(struct pt_regs *regs)
> -{
> -       if (user_mode(regs))
> -               regs->usp = rdusp();
> -}
> -
>  void
>  dik_show_regs(struct pt_regs *regs, unsigned long *r9_15)
>  {
> @@ -186,7 +180,6 @@ do_entArith(unsigned long summary, unsigned long write_mask,
>  {
>         long si_code = FPE_FLTINV;
>
> -       alpha_snapshot_usp(regs);
>         if (summary & 1) {
>                 /* Software-completion summary bit is set, so try to
>                    emulate the instruction.  If the processor supports
> @@ -208,7 +201,6 @@ do_entIF(unsigned long type, struct pt_regs *regs)
>  {
>         int signo, code;
>
> -       alpha_snapshot_usp(regs);
>         if (type == 3) { /* FEN fault */
>                 /* Irritating users can call PAL_clrfen to disable the
>                    FPU for the process.  The kernel will then trap in
>
> --
> 2.55.0
>

Looks good to me, I see the __pad0 is back again in pt_regs
brings back memories from tracking down the stack alignment
issues a while back.

Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>