netdev - Re: csum_partial() on different archs (selftest/bpf)

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Date:   Fri, 13 Nov 2020 14:17:04 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Anders Roxell <anders.roxell@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: csum_partial() on different archs (selftest/bpf)

On Fri, 13 Nov 2020 at 12:34, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 11/13/20 11:36 AM, Björn Töpel wrote:
> > I was running the selftest/bpf on riscv, and had a closer look at one
> > of the failing cases:
> >
> >   #14/p valid read map access into a read-only array 2 FAIL retval
> > 65507 != -29 (run 1/1)
> >
> > The test does a csum_partial() call via a BPF helper. riscv uses the
> > generic implementation. arm64 uses the generic csum_partial() and fail
> > in the same way [1]. arm (32-bit) has a arch specfic implementation,
> > and fail in another way (FAIL retval 131042 != -29) [2].
> >
> > I mimicked the test case in a userland program, comparing the generic
> > csum_partial() to the x86 implementation [3], and the generic and x86
> > implementation does yield a different result.
> >
> > x86     :    -29 : 0xffffffe3
> > generic :  65507 : 0x0000ffe3
> > arm     : 131042 : 0x0001ffe2
> >
> > Who is correct? :-) It would be nice to get rid of this failed case...
> >
>
> There are all the same value :), they all fold to u16  0xFFE3
>
> Maybe the test needs a fix, there is a missing folding.
>

Ah, makes sense. Thank you!

Björn