Re: Problems with csum_partial with misaligned buffers on sh4 platform

[Yoshinori Sato <ysato@users.sourceforge.jp>]

On Sun, 11 Feb 2024 00:12:39 +0900,
Guenter Roeck wrote:
> 
> Hi,
> 
> when running checksum unit tests on sh4 qemu emulations, I get the following
> errors.
> 
>     KTAP version 1
>     # Subtest: checksum
>     # module: checksum_kunit
>     1..5
>     # test_csum_fixed_random_inputs: ASSERTION FAILED at lib/checksum_kunit.c:500
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 53378 (0xd082)
>         ( u64)expec == 33488 (0x82d0)
>     not ok 1 test_csum_fixed_random_inputs
>     # test_csum_all_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:525
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 65281 (0xff01)
>         ( u64)expec == 65280 (0xff00)
>     not ok 2 test_csum_all_carry_inputs
>     # test_csum_no_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:573
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 65535 (0xffff)
>         ( u64)expec == 65534 (0xfffe)
>     not ok 3 test_csum_no_carry_inputs
>     ok 4 test_ip_fast_csum
>     ok 5 test_csum_ipv6_magic
> # checksum: pass:2 fail:3 skip:0 total:5
> 
> The above is with from a little endian system. On a big endian system,
> the test result is as follows.
> 
>     KTAP version 1
>     # Subtest: checksum
>     # module: checksum_kunit
>     1..5
>     # test_csum_fixed_random_inputs: ASSERTION FAILED at lib/checksum_kunit.c:500
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 33488 (0x82d0)
>         ( u64)expec == 53378 (0xd082)
>     not ok 1 test_csum_fixed_random_inputs
>     # test_csum_all_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:525
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 65281 (0xff01)
>         ( u64)expec == 255 (0xff)
>     not ok 2 test_csum_all_carry_inputs
>     # test_csum_no_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:565
>     Expected ( u64)result == ( u64)expec, but
>         ( u64)result == 1020 (0x3fc)
>         ( u64)expec == 0 (0x0)
>     not ok 3 test_csum_no_carry_inputs
>     # test_ip_fast_csum: ASSERTION FAILED at lib/checksum_kunit.c:589
>     Expected ( u64)expected == ( u64)csum_result, but
>         ( u64)expected == 55939 (0xda83)
>         ( u64)csum_result == 33754 (0x83da)
>     not ok 4 test_ip_fast_csum
>     # test_csum_ipv6_magic: ASSERTION FAILED at lib/checksum_kunit.c:617
>     Expected ( u64)expected_csum_ipv6_magic[i] == ( u64)csum_ipv6_magic(saddr, daddr, len, proto, csum), but
>         ( u64)expected_csum_ipv6_magic[i] == 6356 (0x18d4)
>         ( u64)csum_ipv6_magic(saddr, daddr, len, proto, csum) == 43586 (0xaa42)
>     not ok 5 test_csum_ipv6_magic
> # checksum: pass:0 fail:5 skip:0 total:5
> 
> Note that test_ip_fast_csum and test_csum_ipv6_magic fail on all big endian
> systems due to a bug in the test code, unrelated to this problem.
> 
> Analysis shows that the errors are seen only if the buffer is misaligned.
> Looking into arch/sh/lib/checksum.S, I found commit cadc4e1a2b4d2 ("sh:
> Handle calling csum_partial with misaligned data") which seemed to be
> related. Reverting that commit fixes the problem.
> This suggests that something may be wrong with that commit. Alternatively,
> of course, it may be possible that something is wrong with the qemu
> emulation, but that seems unlikely.

I checked that part of the code, and it only uses basic instructions.
If there is a problem with these instructions, other problems should occur,
but I have never seen such a phenomenon.
So I think the culprit is in that commit, not qemu.

I think it's better to use GENERIC_CSUM since the previous code is also
not very efficient.

> Thanks,
> Guenter

-- 
Yosinori Sato