netdev - Re: [RFC] x86/csum: rewrite csum

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Date:   Thu, 11 Nov 2021 18:17:56 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, <x86@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>, <amc96@...f.net>
Subject: Re: [RFC] x86/csum: rewrite csum_partial()

On 11/11/2021 16:52, Eric Dumazet wrote:
> On Thu, Nov 11, 2021 at 8:02 AM Eric Dumazet <edumazet@...gle.com> wrote:
>> Thanks Peter !
>>
>> This is more or less the first version I wrote. (I was doing tests for
>> (len & 32), (len & 16) .. to not have to update len in these blocks.
>>
>> Then, I tried to add an inline version, a la ip_fast_csum() but for IPv6.
>>
>> Then I came up with the version I sent, for some reason my .config had
>> temporarily disabled CONFIG_RETPOLINE,
>> thanks for reminding me this !
>>
>> I also missed this warning anyway :
>> arch/x86/lib/csum-partial_64.o: warning: objtool: csum_partial()+0x2f:
>> unannotated intra-function call
>>
>> I will spend a bit more time on this before sending a V2, thanks again !
> BTW, I could not understand why :
>
>                result = add32_with_carry(result, *(u32 *)buff);
>
> generates this code :
>
>  123: 41 8b 09              mov    (%r9),%ecx
>  126: 89 4d f8              mov    %ecx,-0x8(%rbp)
>  129: 03 45 f8              add    -0x8(%rbp),%eax
>  12c: 83 d0 00              adc    $0x0,%eax

Are you using Clang?  There is a long outstanding code generation bug
where an "rm" constraint is converted to "m" internally.

https://bugs.llvm.org/show_bug.cgi?id=47530

Even a stopgap of pretending "rm" means "r" would result in far better
code, 99% of the time.

> Apparently add32_with_carry() forces the use of use of a temporary in memory
>
> While
>                asm("   addl 0*4(%[src]),%[res]\n"
>                    "   adcl $0,%[res]\n"
>                        : [res] "=r" (result)
>                        : [src] "r" (buff), "[res]" (result)
>                         : "memory");

Just as a minor note about the asm constraints here and elsewhere

    : [res] "=r" (result)
    : "res" (result)

ought to be just [res] "+r" (result).  The result variable really is
read and written by the asm fragments.

~Andrew