linux-kernel - Re: [PATCH] parisc: Fix csum_ipv6

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Date: Mon, 19 Feb 2024 22:11:54 +0100
From: Helge Deller <deller@....de>
To: Charlie Jenkins <charlie@...osinc.com>, Helge Deller <deller@...nel.org>,
 Guenter Roeck <linux@...ck-us.net>
Cc: "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
 linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
 Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH] parisc: Fix csum_ipv6_magic on 64-bit systems

On 2/17/24 04:00, Charlie Jenkins wrote:
> On Fri, Feb 16, 2024 at 01:38:51PM +0100, Helge Deller wrote:
>> * Guenter Roeck <linux@...ck-us.net>:
>>> hppa 64-bit systems calculates the IPv6 checksum using 64-bit add
>>> operations. The last add folds protocol and length fields into the 64-bit
>>> result. While unlikely, this operation can overflow. The overflow can be
>>> triggered with a code sequence such as the following.
>>>
>>> 	/* try to trigger massive overflows */
>>> 	memset(tmp_buf, 0xff, sizeof(struct in6_addr));
>>> 	csum_result = csum_ipv6_magic((struct in6_addr *)tmp_buf,
>>> 				      (struct in6_addr *)tmp_buf,
>>> 				      0xffff, 0xff, 0xffffffff);
>>>
>>> Fix the problem by adding any overflows from the final add operation into
>>> the calculated checksum. Fortunately, we can do this without additional
>>> cost by replacing the add operation used to fold the checksum into 32 bit
>>> with "add,dc" to add in the missing carry.
>>
>>
>> Gunter,
>>
>> Thanks for your patch for 32- and 64-bit systems.
>> But I think it's time to sunset the parisc inline assembly for ipv4/ipv6
>> checksumming.
>> The patch below converts the code to use standard C-coding (taken from the
>> s390 header file) and it survives the v8 lib/checksum patch.
>>
>> Opinions?
>> [...]

> We can do better than this! By inspection this looks like a performance
> regression.
> [...]
> Similar story again here where the add with carry is not well translated
> into C, resulting in significantly worse assembly. Using __u64 seems to
> be a big contributing factor for why the 32-bit assembly is worse.
>
> I am not sure the best way to represent this in a clean way in C.
>
> add with carry is not well understood by GCC 12.3 it seems. These
> functions are generally heavily optimized on every architecture, so I
> think it is worth it to default to assembly if you aren't able to
> achieve comparable performance in C.

Thanks a lot for your analysis!!!
I've now reverted my change to switch to generic code and applied
the 3 suggested patches from Guenter which fix the hppa assembly.
Let's see how they behave in the for-next git tree during the next few days.

Helge