[PATCH] Align the scratch allocation to 64 bytes

[Xiang W]

在 2025-01-09四的 16:34 -0800，Chao-ying Fu写道：
> We increase the scratch allocation alignment to 64 bytes
> as the cache line size, to avoid two atomic variables from
> the same cache line that may cause livelock on some platforms.
> ---
>  lib/sbi/sbi_scratch.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> index ccbbc68..6b161bc 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -14,6 +14,9 @@
>  #include <sbi/sbi_scratch.h>
>  #include <sbi/sbi_string.h>
>  
> +/* Minimum size and alignment of scratch allocations */
> +#define SCRATCH_ALLOC_ALIGN 64
This appears to be a platform-specific need and should not affect all platforms.
Can be like this:

#ifndef SCRATCH_ALLOC_ALIGN
#define SCRATCH_ALLOC_ALIGN __SIZEOF_POINTER__
#endif

Regards,
Xiang W
> +
>  u32 last_hartindex_having_scratch = 0;
>  u32 hartindex_to_hartid_table[SBI_HARTMASK_MAX_BITS + 1] = { -1U };
>  struct sbi_scratch *hartindex_to_scratch_table[SBI_HARTMASK_MAX_BITS + 1] = { 0 };
> @@ -70,8 +73,12 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
>  	if (!size)
>  		return 0;
>  
> -	size += __SIZEOF_POINTER__ - 1;
> -	size &= ~((unsigned long)__SIZEOF_POINTER__ - 1);
> +	/*
> +	 * We let the allocation align to 64 bytes to avoid livelock on
> +	 * certain platforms due to atomic variables from the same cache line.
> +	 */
> +	size += SCRATCH_ALLOC_ALIGN - 1;
> +	size &= ~((unsigned long)SCRATCH_ALLOC_ALIGN - 1);
>  
>  	spin_lock(&extra_lock);
>  
> -- 
> 2.47.1
> 
>