Re: [PATCH v3] arm64: lib: accelerate do_csum with NEON instruction

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Fri, 18 Jan 2019 at 02:07, huanglingyan (A) <huanglingyan2@huawei.com> wrote:
>
>
> On 2019/1/17 0:46, Will Deacon wrote:
> > On Wed, Jan 09, 2019 at 10:03:05AM +0800, huanglingyan (A) wrote:
> >> On 2019/1/8 21:54, Will Deacon wrote:
> >>> [re-adding Ard and LAKML -- not sure why the headers are so munged]
> >>>
> >>> On Mon, Jan 07, 2019 at 10:38:55AM +0800, huanglingyan (A) wrote:
> >>>> On 2019/1/6 16:26, Ard Biesheuvel wrote:
> >>>>     Please change this into
> >>>>
> >>>>     if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) &&
> >>>>         len >= CSUM_NEON_THRESHOLD &&
> >>>>         may_use_simd()) {
> >>>>             kernel_neon_begin();
> >>>>             res = do_csum_neon(buff, len);
> >>>>             kernel_neon_end();
> >>>>         }
> >>>>
> >>>>     and drop the intermediate do_csum_arm()
> >>>>
> >>>>
> >>>>         +               return do_csum_arm(buff, len);
> >>>>         +#endif  /* CONFIG_KERNEL_MODE_NEON */
> >>>>
> >>>>     No else? What happens if len < CSUM_NEON_THRESHOLD ?
> >>>>
> >>>>
> >>>>         +#undef do_csum
> >>>>
> >>>>     Can we drop this?
> >>>>
> >>>> Using NEON instructions will bring some costs. The spending maybe introduced
> >>>> when reservering/restoring
> >>>> neon registers with kernel_neon_begin()/kernel_neon_end(). Therefore NEON code
> >>>> is Only used when
> >>>> the length exceeds CSUM_NEON_THRESHOLD. General do csum() codes in lib/
> >>>> checksum.c will be used in
> >>>> shorter length. To achieve this goal, I use the "#undef do_csum" in else clause
> >>>> to have the oppotunity to
> >>>> utilize the general codes.
> >>> I don't think that's how it works :/
> >>>
> >>> Before we get deeper into the implementation, please could you justify the
> >>> need for a CPU-optimised checksum implementation at all? I thought this was
> >>> usually offloaded to the NIC?
> >>>
> >>> Will
> >>>
> >>> .
> >> This problem is introduced when testing Intel x710 network card on my ARM server.
> >> Ip forward is set for ease of testing. Then send lots of packages to server by Tesgine
> >> machine and then receive.
> > In the marketing blurb, that card boasts:
> >
> >   `Tx/Rx IP, SCTP, TCP, and UDP checksum offloading (IPv4, IPv6) capabilities'
> >
> > so we shouldn't need to run this on the CPU. Again, I'm not keen to optimise
> > this given that it /really/ shouldn't be used on arm64 machines that care
> > about network performance.
> >
> > Will
> >
> > .
> Yeah, you are right. Checksum is usually done in network card which is told by
> someone familiar with NIC. However, it may be used in testing scenaries and
> some primary network cards. I think it's no harm to optimize this code while
> other ARCHs have their own optimized versions.
> >
>

I disagree. If this code path is never exercised, we should not
include it. We can revisit this decision when there is a use case
where the checksumming performance is an actual bottleneck.

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