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 16:25:16 +0100	[thread overview]
Message-ID: <YY6HXBK7UN4YqBJm@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CANn89iJNvxatTTcHvzNKuUu2HyNfH=O7XesA3pHUwfn4Qy=pJQ@mail.gmail.com>

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@infradead.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