linux-kernel - Re: [PATCH 3/4] RISC-V: Fix L1_CACHE

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Date:   Wed, 13 Jan 2021 21:09:34 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Atish Patra <Atish.Patra@....com>
CC:     linux-kernel@...r.kernel.org, Atish Patra <Atish.Patra@....com>,
        aou@...s.berkeley.edu, akpm@...ux-foundation.org,
        Anup Patel <Anup.Patel@....com>, ardb@...nel.org,
        linux-riscv@...ts.infradead.org, rppt@...nel.org,
        Paul Walmsley <paul.walmsley@...ive.com>, mick@....forth.gr
Subject:     Re: [PATCH 3/4] RISC-V: Fix L1_CACHE_BYTES for RV32

On Thu, 07 Jan 2021 01:26:51 PST (-0800), Atish Patra wrote:
> SMP_CACHE_BYTES/L1_CACHE_BYTES should be defined as 32 instead of
> 64 for RV32. Otherwise, there will be hole of 32 bytes with each memblock
> allocation if it is requested to be aligned with SMP_CACHE_BYTES.
>
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/include/asm/cache.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h
> index 9b58b104559e..c9c669ea2fe6 100644
> --- a/arch/riscv/include/asm/cache.h
> +++ b/arch/riscv/include/asm/cache.h
> @@ -7,7 +7,11 @@
>  #ifndef _ASM_RISCV_CACHE_H
>  #define _ASM_RISCV_CACHE_H
>
> +#ifdef CONFIG_64BIT
>  #define L1_CACHE_SHIFT		6
> +#else
> +#define L1_CACHE_SHIFT		5
> +#endif
>
>  #define L1_CACHE_BYTES		(1 << L1_CACHE_SHIFT)

Should we not instead just

#define SMP_CACHE_BYTES L1_CACHE_BYTES

like a handful of architectures do?

The cache size is sort of fake here, as we don't have any non-coherent
mechanisms, but IIRC we wrote somewhere that it's recommended to have 64-byte
cache lines in RISC-V implementations as software may assume that for
performance reasons.  Not really a strong reason, but I'd prefer to just make
these match.