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From:   Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Fri, 26 Nov 2021 17:04:20 -0600
Message-ID: <CAFUsyfJmpFFzuMhHrH+oBVzcHggW0QZM9dvXtPQW88kAw_2_BQ@mail.gmail.com>
Subject: Re: [tip:x86/core 1/1] arch/x86/um/../lib/csum-partial_64.c:98:12:
 error: implicit declaration of function 'load_unaligned_zeropad'
To:     David Laight <David.Laight@aculab.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        "alexanderduyck@fb.com" <alexanderduyck@fb.com>,
        "kbuild-all@lists.01.org" <kbuild-all@lists.01.org>,
        open list <linux-kernel@vger.kernel.org>,
        "linux-um@lists.infradead.org" <linux-um@lists.infradead.org>,
        "lkp@intel.com" <lkp@intel.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        X86 ML <x86@kernel.org>
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On Fri, Nov 26, 2021 at 4:41 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Eric Dumazet
> > Sent: 26 November 2021 18:10
> ...
> > > AFAICT (from a pdf) bswap32() and ror(x, 8) are likely to be
> > > the same speed but may use different execution units.
>
> The 64bit shifts/rotates are also only one clock.
> It is the bswap64 that can be two.
>
> > > Intel seem so have managed to slow down ror(x, %cl) to 3 clocks
> > > in sandy bridge - and still not fixed it.
> > > Although the compiler might be making a pigs-breakfast of the
> > > register allocation when you tried setting 'odd = 8'.
> > >
> > > Weeks can be spent fiddling with this code :-(
> >
> > Yes, and in the end, it won't be able to compete with  a
> > specialized/inlined ipv6_csum_partial()
>
> I bet most of the gain comes from knowing there is a non-zero
> whole number of 32bit words.
> The pesky edge conditions cost.
>
> And even then you need to get it right!
> The one for summing the 5-word IPv4 header is actually horrid
> on Intel cpu prior to Haswell because 'adc' has a latency of 2.
> On Sandy bridge the carry output is valid on the next clock,
> so adding to alternate registers doubles throughput.
> (That could easily be done in the current function and will
> make a big different on those cpu.)
>
> But basically the current generic code has the loop unrolled
> further than is necessary for modern (non-atom) cpu.
> That just adds more code outside the loop.
>
> I did managed to get 12 bytes/clock using adco/adox with only
> 32 bytes each iteration.
> That will require aligned buffers.
>
> Alignment won't matter for 'adc' loops because there are two
> 'memory read' units - but there is the elephant:
>
> Sandy bridge Cache bank conflicts
> Each consecutive 128 bytes, or two cache lines, in the data cache is divided
> into 8 banks of 16 bytes each. It is not possible to do two memory reads in
> the same clock cycle if the two memory addresses have the same bank number,
> i.e. if bit 4 - 6 in the two addresses are the same.
>         ; Example 9.5. Sandy bridge cache bank conflict
>         mov eax, [rsi] ; Use bank 0, assuming rsi is divisible by 40H
>         mov ebx, [rsi+100H] ; Use bank 0. Cache bank conflict
>         mov ecx, [rsi+110H] ; Use bank 1. No cache bank conflict
>
> That isn't a problem on Haswell, but it is probably worth ordering
> the 'adc' in the loop to reduce the number of conflicts.
> I didn't try to look for that though.
> I only remember testing aligned buffers on Sandy/Ivy bridge.
> Adding to alternate registers helped no end.

Cant that just be solved by having the two independent adcx/adox chains work
from region that are 16+ bytes apart? For 40 byte ipv6 header it will be simple.
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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