linux-kernel - Re: [PATCH 1/3] riscv: optimized memcpy

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Date:   Tue, 15 Jun 2021 15:44:48 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Bin Meng <bmeng.cn@...il.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atish.patra@....com>,
        Emil Renner Berthing <kernel@...il.dk>,
        Akira Tsukamoto <akira.tsukamoto@...il.com>,
        Drew Fustini <drew@...gleboard.org>
Subject: Re: [PATCH 1/3] riscv: optimized memcpy

On Tue, Jun 15, 2021 at 3:18 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Bin Meng
> > Sent: 15 June 2021 14:09
> >
> > On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight@...lab.com> wrote:
> > >
> ...
> > > I'm surprised that the C loop:
> > >
> > > > +             for (; count >= bytes_long; count -= bytes_long)
> > > > +                     *d.ulong++ = *s.ulong++;
> > >
> > > ends up being faster than the ASM 'read lots' - 'write lots' loop.
> >
> > I believe that's because the assembly version has some unaligned
> > access cases, which end up being trap-n-emulated in the OpenSBI
> > firmware, and that is a big overhead.
>
> Ah, that would make sense since the asm user copy code
> was broken for misaligned copies.
> I suspect memcpy() was broken the same way.
>
> I'm surprised IP_NET_ALIGN isn't set to 2 to try to
> avoid all these misaligned copies in the network stack.
> Although avoiding 8n+4 aligned data is rather harder.
>

That's up to the network driver, indeed I have a patch already for the
BeagleV one:

https://lore.kernel.org/netdev/20210615012107.577ead86@linux.microsoft.com/T/

> Misaligned copies are just best avoided - really even on x86.
> The 'real fun' is when the access crosses TLB boundaries.
>

-- 
per aspera ad upstream