linux-kernel - Re: x86/csum: Remove unnecessary odd handling

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Date:   Wed, 28 Jun 2023 10:32:12 -0500
From:   Noah Goldstein <goldstein.w.n@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        edumazet@...gle.com, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: x86/csum: Remove unnecessary odd handling

On Wed, Jun 28, 2023 at 4:12 AM Borislav Petkov <bp@...en8.de> wrote:
>
> + Linus who's been poking at this yesterday.
>
Linus, if you're planning a patch and want to just integrate the codes
here I'm happy drop this patch

> + lkml. Please always CC lkml when sending patches.

Ah, will do so in the future.
>
> On Tue, Jun 27, 2023 at 09:06:57PM -0500, Noah Goldstein wrote:
> > The special case for odd aligned buffers is unnecessary and mostly
> > just adds overhead. Aligned buffers is the expectations, and even for
> > unaligned buffer, the only case that was helped is if the buffer was
> > 1-byte from word aligned which is ~1/7 of the cases. Overall it seems
> > highly unlikely to be worth to extra branch.
> >
> > It was left in the previous perf improvement patch because I was
> > erroneously comparing the exact output of `csum_partial(...)`, but
> > really we only need `csum_fold(csum_partial(...))` to match so its
> > safe to remove.
> >
> > All csum kunit tests pass.
> >
> > Signed-off-by: Noah Goldstein <goldstein.w.n@...il.com>
> > ---
> >  arch/x86/lib/csum-partial_64.c | 37 ++--------------------------------
> >  1 file changed, 2 insertions(+), 35 deletions(-)
> >
> > diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
> > index cea25ca8b8cf..d06112e98893 100644
> > --- a/arch/x86/lib/csum-partial_64.c
> > +++ b/arch/x86/lib/csum-partial_64.c
> > @@ -11,28 +11,6 @@
> >  #include <asm/checksum.h>
> >  #include <asm/word-at-a-time.h>
> >
> > -static inline unsigned short from32to16(unsigned a)
> > -{
> > -     unsigned short b = a >> 16;
> > -     asm("addw %w2,%w0\n\t"
> > -         "adcw $0,%w0\n"
> > -         : "=r" (b)
> > -         : "0" (b), "r" (a));
> > -     return b;
> > -}
> > -
> > -static inline __wsum csum_tail(u64 temp64, int odd)
> > -{
> > -     unsigned int result;
> > -
> > -     result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
> > -     if (unlikely(odd)) {
> > -             result = from32to16(result);
> > -             result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
> > -     }
> > -     return (__force __wsum)result;
> > -}
> > -
> >  /*
> >   * Do a checksum on an arbitrary memory area.
> >   * Returns a 32bit checksum.
> > @@ -47,17 +25,6 @@ static inline __wsum csum_tail(u64 temp64, int odd)
> >  __wsum csum_partial(const void *buff, int len, __wsum sum)
> >  {
> >       u64 temp64 = (__force u64)sum;
> > -     unsigned odd;
> > -
> > -     odd = 1 & (unsigned long) buff;
> > -     if (unlikely(odd)) {
> > -             if (unlikely(len == 0))
> > -                     return sum;
> > -             temp64 = ror32((__force u32)sum, 8);
> > -             temp64 += (*(unsigned char *)buff << 8);
> > -             len--;
> > -             buff++;
> > -     }
> >
> >       /*
> >        * len == 40 is the hot case due to IPv6 headers, but annotating it likely()
> > @@ -73,7 +40,7 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
> >                   "adcq $0,%[res]"
> >                   : [res] "+r"(temp64)
> >                   : [src] "r"(buff), "m"(*(const char(*)[40])buff));
> > -             return csum_tail(temp64, odd);
> > +             return add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
> >       }
> >       if (unlikely(len >= 64)) {
> >               /*
> > @@ -143,7 +110,7 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
> >                   : [res] "+r"(temp64)
> >                   : [trail] "r"(trail));
> >       }
> > -     return csum_tail(temp64, odd);
> > +     return add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
> >  }
> >  EXPORT_SYMBOL(csum_partial);
> >
> > --
> > 2.34.1
> >
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette