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

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Message-ID: <YY4wPgyt65Q6WOdK@hirez.programming.kicks-ass.net>
Date:   Fri, 12 Nov 2021 10:13:34 +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 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().