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

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Date:   Fri, 12 Nov 2021 16:25:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
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 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