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

[Will Deacon <will.deacon@arm.com>]

[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:
> 在 2019/1/6 16:26, Ard Biesheuvel 写道:
>     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

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