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

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Date:   Thu, 11 Nov 2021 11:02:48 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>, amc96@...f.net
Subject: Re: [RFC] x86/csum: rewrite csum_partial()

On Thu, Nov 11, 2021 at 10:18 AM Andrew Cooper
<andrew.cooper3@...rix.com> wrote:
>
> 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.
>

Yes, this is what I realized later. This is a clang bug.

> 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.

Agreed

>
> > 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.

Thanks for the tip !