[PATCH] lib: sbi: Issue a PAUSE hint between spins in spin_lock()

[Anup Patel]

On Tue, Apr 23, 2024 at 9:13 PM Christoph Müllner
<christoph.muellner at vrull.eu> wrote:
>
> RISC-V's Zihintpause extension includes a PAUSE instruction that
> is designed to reduce engergy consumption while executing spin-wait
> code sequences.  It is encoded as a HINT, which means that it
> behaves like a NOP on systems that don't implement Zihintpause.
>
> Binutils supports Zihintpause since 2.36, so this patch uses
> the ".insn" directive to keep the code compatible with older
> toolchains.
>
> Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
> ---
>  lib/sbi/riscv_locks.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/lib/sbi/riscv_locks.c b/lib/sbi/riscv_locks.c
> index acab776..3c26a77 100644
> --- a/lib/sbi/riscv_locks.c
> +++ b/lib/sbi/riscv_locks.c
> @@ -61,7 +61,8 @@ void spin_lock(spinlock_t *lock)
>                 "1:     and     %2, %0, %5\n"
>                 "       beq     %1, %2, 2f\n"
>
> -               /* If not, then spin on the lock. */
> +               /* If not, then issue a PAUSE hint and spin on the lock. */
> +               "       .insn   i 0x0f, 0, x0, x0, 0x010\n"

There are a lot of platforms without Zihintpause so we need
instruction patching here.

>                 "       lw      %0, %3\n"
>                 RISCV_ACQUIRE_BARRIER
>                 "       j       1b\n"
> --
> 2.44.0
>
>
> --
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Regards,
Anup