Re: [PATCH v1] x86/csum: rewrite csum_partial()

From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <edumazet@google.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, netdev <netdev@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [PATCH v1] x86/csum: rewrite csum_partial()
Date: Fri, 12 Nov 2021 10:13:34 +0100	[thread overview]
Message-ID: <YY4wPgyt65Q6WOdK@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CANn89iJAakUCC6UuUHSozT9wz7_rrgrRq3dv+hXJ1FL_DCZHyA@mail.gmail.com>

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().