Message-ID: <11842961-91c2-40bf-9134-7528d30137f3@roeck-us.net>
Date: Fri, 1 Mar 2024 12:47:18 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 Charlie Jenkins <charlie@...osinc.com>,
 David Laight <David.Laight@...lab.com>, Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Helge Deller <deller@....de>,
 "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 Parisc List <linux-parisc@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Palmer Dabbelt <palmer@...osinc.com>,
 Linux ARM <linux-arm-kernel@...ts.infradead.org>,
 Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH v10] lib: checksum: Use aligned accesses for ip_fast_csum
 and csum_ipv6_magic tests

On 3/1/24 08:24, Guenter Roeck wrote:
> On 2/29/24 22:46, Christophe Leroy wrote:
>>
>>
>> Le 26/02/2024 à 17:44, Guenter Roeck a écrit :
>>> On 2/26/24 03:34, Christophe Leroy wrote:
>>>>
>>>>
>>>> Le 23/02/2024 à 23:11, Charlie Jenkins a écrit :
>>>>> The test cases for ip_fast_csum and csum_ipv6_magic were not properly
>>>>> aligning the IP header, which were causing failures on architectures
>>>>> that do not support misaligned accesses like some ARM platforms. To
>>>>> solve this, align the data along (14 + NET_IP_ALIGN) bytes which is the
>>>>> standard alignment of an IP header and must be supported by the
>>>>> architecture.
>>>>
>>>> I'm still wondering what we are really trying to fix here.
>>>>
>>>> All other tests are explicitely testing that it works with any alignment.
>>>>
>>>> Shouldn't ip_fast_csum() and csum_ipv6_magic() work for any alignment as
>>>> well ? I would expect it, I see no comment in arm code which explicits
>>>> that assumption around those functions.
>>>>
>>>> Isn't the problem only the following line, because csum_offset is
>>>> unaligned ?
>>>>
>>>> csum = *(__wsum *)(random_buf + i + csum_offset);
>>>>
>>>> Otherwise, if there really is an alignment issue for the IPv6 source or
>>>> destination address, isn't it enough to perform a 32 bits alignment ?
>>>>
>>>
>>> It isn't just arm.
>>>
>>> Question should be what alignments the functions are supposed to be able
>>> to handle, not what they are optimized for. If byte and/or half word
>>> alignments
>>> are expected to be supported, there is still architecture code which would
>>> have to be fixed. Unaligned accesses are known to fail on hppa64/parisc64
>>> and on sh4, for example. If unaligned accesses are expected to be handled,
>>> it would probably make sense to add a separate test case, though, to
>>> clarify
>>> that the test fails due to alignment issues, not due to input parameters.
>>>
>>
>> When you say "Unaligned accesses are known to fail on hppa64/parisc64
>> and on sh4", do you mean unaligned accesses in general or do you mean
>> ip_fast_csum() with unaligned ip header and csum_ipv6_magic() with
>> unaligned source and dest addresses ?
>>
>> Because later in this thread it is said that only ARM and NIOS2
>> potentially have an issue.
>>
>> And when you say "unaligned", to what level is that ? Is it 4-bytes
>> alignment or more or less ?
>>
> 
> This e-mail chain is getting too long. Here is an attempt of a quick summary.
> 
> - Someone else suggested that unaligned accesses with nios2 should fail.
>    I since then tested and found that they pass at least for the checksum tests,
>    while dumping "unaligned access" messages into the kernel log. Other tests
>    (specifically gso) cause crashes, but that is unrelated.
> 
> - checksum tests on sh4 fail for unaligned data because of a bug introduced
>    to the architecture's checksum core with commit cadc4e1a2b4d ("sh: Handle
>    calling csum_partial with misaligned data"). The tests pass after reverting
>    that patch. I reported this, but that revert (or a fix of the problem it
>    introduced) has not been applied to linux-next.
> 
> - Checksum tests on unaligned data fail on parisc in mainline due to a number
>    of bugs in checksum assembler code (and with upstream qemu due to a bug in
>    qemu's hppa64 emulation). All those issues should by now be fixed in linux-next.
>    For reference, the following patches (SHAs from next-20240301) are needed to fix
>    the known problems:
>      0568b6f0d863 parisc: Strip upper 32 bit of sum in csum_ipv6_magic for 64-bit builds
>      4b75b12d7050 parisc: Fix csum_ipv6_magic on 64-bit systems
>      4408ba75e4ba parisc: Fix csum_ipv6_magic on 32-bit systems
>      a2abae8f0b63 parisc: Fix ip_fast_csum
>    qemu (v8.2 and later) needs
>      https://lore.kernel.org/all/20240217015811.1975411-1-linux@roeck-us.net/T/
>    for the hppa64/parisc64 tests to work with qemu.
> 
> - Checksum tests on unaligned data cause a crash on arm systems with "thumb"
>    instruction set enabled (such as mps2_defconfig and an385). I didn't bother
>    checking if the crash is with 1-byte or 2-byte alignment.
> 
> - There used to be a crash with checksum tests on m68k because of word alignment
>    which the implementation of the unit tests for csum_ipv6_magic() did not take
>    into account (this is fixed by the padding in struct csum_ipv6_magic_data).
>    I don't know if this patch is needed to fix that problem or if it was since
>    fixed differently.
> 
> I hope that covers everything. As I said above, the chain is getting long
> and I may have missed something.
> 

I knew I missed something. I forgot to mention upstream commit d55347bfe4e6
("MIPS: Add 'memory' clobber to csum_ipv6_magic() inline assembler") witch
fixed problems with csum_ipv6_magic() for mips.

Guenter

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