linux-kernel - Re: [PATCH] samples/bpf: Fix test_maps/bpf_get_next

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Date:	Fri, 23 Jan 2015 15:13:46 +0100
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Alexei Starovoitov <ast@...nel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] samples/bpf: Fix test_maps/bpf_get_next_key() test

On Thu, 22 Jan 2015 09:32:43 -0800
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> On Thu, Jan 22, 2015 at 8:01 AM, Michael Holzheu
> <holzheu@...ux.vnet.ibm.com> wrote:
> > Looks like the "test_maps" test case expects to get the keys in
> > the wrong order when iterating over the elements:
> >
> > test_maps: samples/bpf/test_maps.c:79: test_hashmap_sanity: Assertion
> > `bpf_get_next_key(map_fd, &key, &next_key) == 0 && next_key == 2' failed.
> > Aborted
> >
> > Fix this and test for the correct order.
> 
> that will break this test on x86...
> we need to understand first why the order of two elements
> came out different on s390...
> Could it be that jhash() produced different hash for the same
> values on x86 vs s390 ?

Yes I think jhash() produces different results for input > 12 bytes
on big and little endian machines because of the following code
in include/linux/jhash.h:

        while (length > 12) {
                a += __get_unaligned_cpu32(k);
                b += __get_unaligned_cpu32(k + 4);
                c += __get_unaligned_cpu32(k + 8);
                __jhash_mix(a, b, c);
                length -= 12;
                k += 12;
        }

The contents of "k" is directly used as u32 and the result
of "__get_unaligned_cpu32(k)" is different for big and
little endian.

Michael

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