Re: [PATCH 1/3] riscv: optimized memcpy

[Bin Meng]

On Tue, Jun 15, 2021 at 9:18 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Bin Meng
> > Sent: 15 June 2021 14:09
> >
> > On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight@xxxxxxxxxx> 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.
>

Yes, Gary Guo sent one patch long time ago against the broken assembly
version, but that patch was still not applied as of today.
https://patchwork.kernel.org/project/linux-riscv/patch/20210216225555.4976-1-gary@xxxxxxxxxxx/

I suggest Matteo re-test using Gary's version.

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

Regards,
Bin