linux-kernel - Re: [PATCH 3/4] RISC-V: Fix L1_CACHE_BYTES for RV32

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Message-ID: <CAOnJCU+mCPwbeOQpmHmu3ar_17otmgftiKHLL+Z4_nExpj0=cA@mail.gmail.com>
Date:   Thu, 14 Jan 2021 10:33:01 -0800
From:   Atish Patra <atishp@...shpatra.org>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Atish Patra <Atish.Patra@....com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <Anup.Patel@....com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Nick Kossifidis <mick@....forth.gr>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH 3/4] RISC-V: Fix L1_CACHE_BYTES for RV32

On Wed, Jan 13, 2021 at 9:10 PM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> 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 generic code already defines it that way in include/linux/cache.h

> 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.
>

If it is documented somewhere in the kernel, we should update that. I
think SMP_CACHE_BYTES being 64
actually degrades the performance as there will be a fragmented memory
blocks with 32 bit bytes gap wherever
SMP_CACHE_BYTES is used as an alignment requirement.

In addition to that, Geert Uytterhoeven mentioned some panic on vex32
without this patch.
I didn't see anything in Qemu though.

> _______________________________________________
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-- 
Regards,
Atish

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