Re: [PATCH] riscv: Add WFI to secondary hart spinwait loop

[Nam Cao]

Adriano Vero <litaliano00.contact@xxxxxxxxx> writes:

> The .Lwait_for_cpu_up loop in the RISCV_BOOT_SPINWAIT path busy-polls
> __cpu_spinwait_stack_pointer and __cpu_spinwait_task_pointer, burning
> power on all non-boot harts while they wait for the primary hart to
> complete early boot setup.
>
> Add a WFI instruction before each polling iteration to allow the
> hardware to enter a low-power state while waiting. Per the RISC-V
> privileged specification, WFI wakes on any pending interrupt even
> with global interrupts disabled (SIE=0), and implementations are
> permitted to treat it as a NOP, so this is safe in all contexts.
>
> The same pattern is already used in .Lsecondary_park in the same
> file.
>
> Signed-off-by: Adriano Vero <litaliano00.contact@xxxxxxxxx>
> ---
>  arch/riscv/kernel/head.S | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 9c99c5ad6..ca208da7c 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -385,7 +385,14 @@ SYM_CODE_START(_start_kernel)
>  	 * get far enough along the boot process that it should continue.
>  	 */
>  .Lwait_for_cpu_up:
> -	/* FIXME: We should WFI to save some energy here. */
> +	/*
> +	 * Wait for the boot hart to populate the stack and task pointers.
> +	 * Use WFI to avoid burning power in a busy-wait loop. Per the
> +	 * RISC-V privileged spec, WFI wakes on a pending interrupt even
> +	 * with global interrupts disabled (e.g. SIE=0), and implementations
> +	 * are permitted to treat it as a NOP, so this is always safe.
> +	 */
> +	wfi
>  	REG_L sp, (a1)
>  	REG_L tp, (a2)
>  	beqz sp, .Lwait_for_cpu_up
> -- 
> 2.53.0
>
>
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