netdev - Re: [PATCH v1] x86/csum: rewrite csum

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Message-ID: <CANn89iLpjDsSzLCMz6w8TO5zu1N6HoWM3sgzQevv=zpMYaOt8g@mail.gmail.com>
Date:   Fri, 12 Nov 2021 07:37:48 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v1] x86/csum: rewrite csum_partial()

On Fri, Nov 12, 2021 at 7:25 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Nov 12, 2021 at 06:21:38AM -0800, Eric Dumazet wrote:
> > On Fri, Nov 12, 2021 at 1:13 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Thu, Nov 11, 2021 at 02:30:50PM -0800, Eric Dumazet wrote:
> > > > > For values 7 through 1 I wonder if you wouldn't be better served by
> > > > > just doing a single QWORD read and a pair of shifts. Something along
> > > > > the lines of:
> > > > >     if (len) {
> > > > >         shift = (8 - len) * 8;
> > > > >         temp64 = (*(unsigned long)buff << shift) >> shift;
> > > > >         result += temp64;
> > > > >         result += result < temp64;
> > > > >     }
> > > >
> > > > Again, KASAN will not be happy.
> > >
> > > If you do it in asm, kasan will not know, so who cares :-) as long as
> > > the load is aligned, loading beyond @len shouldn't be a problem,
> > > otherwise there's load_unaligned_zeropad().
> >
> > OK, but then in this case we have to align buff on qword boundary,
> > or risk crossing page boundary.
>
> Read the above, use load_unaligned_zeropad(), it's made for exactly that
> case.
>
> Slightly related, see:
>
>   https://lkml.kernel.org/r/20211110101326.141775772@infradead.org

Ah right you are, thanks a lot !