Re: [PATCH v4] net: fix checksum with unaligned buffer

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2022-06-23 14:51, Morten Brørup wrote:
>> From: Morten Brørup [mailto:mb@smartsharesystems.com]
>> Sent: Thursday, 23 June 2022 14.39
>>
>> With this patch, the checksum can be calculated on an unaligned buffer.
>> I.e. the buf parameter is no longer required to be 16 bit aligned.
>>
>> The checksum is still calculated using a 16 bit aligned pointer, so the
>> compiler can auto-vectorize the function's inner loop.
>>
>> When the buffer is unaligned, the first byte of the buffer is handled
>> separately. Furthermore, the calculated checksum of the buffer is byte
>> shifted before being added to the initial checksum, to compensate for
>> the
>> checksum having been calculated on the buffer shifted by one byte.
>>
>> v4:
>> * Add copyright notice.
>> * Include stdbool.h (Emil Berg).
>> * Use RTE_PTR_ADD (Emil Berg).
>> * Fix one more typo in commit message. Is 'unligned' even a word?
>> v3:
>> * Remove braces from single statement block.
>> * Fix typo in commit message.
>> v2:
>> * Do not assume that the buffer is part of an aligned packet buffer.
>>
>> Bugzilla ID: 1035
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
>> ---
>>   lib/net/rte_ip.h | 32 +++++++++++++++++++++++++++-----
>>   1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
>> index b502481670..738d643da0 100644
>> --- a/lib/net/rte_ip.h
>> +++ b/lib/net/rte_ip.h
>> @@ -3,6 +3,7 @@
>>    *      The Regents of the University of California.
>>    * Copyright(c) 2010-2014 Intel Corporation.
>>    * Copyright(c) 2014 6WIND S.A.
>> + * Copyright(c) 2022 SmartShare Systems.
>>    * All rights reserved.
>>    */
>>
>> @@ -15,6 +16,7 @@
>>    * IP-related defines
>>    */
>>
>> +#include <stdbool.h>
>>   #include <stdint.h>
>>
>>   #ifdef RTE_EXEC_ENV_WINDOWS
>> @@ -162,20 +164,40 @@ __rte_raw_cksum(const void *buf, size_t len,
>> uint32_t sum)
>>   {
>>   	/* extend strict-aliasing rules */
>>   	typedef uint16_t __attribute__((__may_alias__)) u16_p;
>> -	const u16_p *u16_buf = (const u16_p *)buf;
>> -	const u16_p *end = u16_buf + len / sizeof(*u16_buf);
>> +	const u16_p *u16_buf;
>> +	const u16_p *end;
>> +	uint32_t bsum = 0;
>> +	const bool unaligned = (uintptr_t)buf & 1;
>> +
>> +	/* if buffer is unaligned, keeping it byte order independent */
>> +	if (unlikely(unaligned)) {
>> +		uint16_t first = 0;
>> +		if (unlikely(len == 0))
>> +			return 0;
>> +		((unsigned char *)&first)[1] = *(const unsigned char *)buf;
>> +		bsum += first;
>> +		buf = RTE_PTR_ADD(buf, 1);
>> +		len--;
>> +	}
>>
>> +	/* aligned access for compiler auto-vectorization */

The compiler will be able to auto vectorize even unaligned accesses, 
just with different instructions. From what I can tell, there's no 
performance impact, at least not on the x86_64 systems I tried on.

I think you should remove the first special case conditional and use 
memcpy() instead of the cumbersome __may_alias__ construct to retrieve 
the data.

>> +	u16_buf = (const u16_p *)buf;
>> +	end = u16_buf + len / sizeof(*u16_buf);
>>   	for (; u16_buf != end; ++u16_buf)
>> -		sum += *u16_buf;
>> +		bsum += *u16_buf;
>>
>>   	/* if length is odd, keeping it byte order independent */
>>   	if (unlikely(len % 2)) {
>>   		uint16_t left = 0;
>>   		*(unsigned char *)&left = *(const unsigned char *)end;
>> -		sum += left;
>> +		bsum += left;
>>   	}
>>
>> -	return sum;
>> +	/* if buffer is unaligned, swap the checksum bytes */
>> +	if (unlikely(unaligned))
>> +		bsum = (bsum & 0xFF00FF00) >> 8 | (bsum & 0x00FF00FF) << 8;
>> +
>> +	return sum + bsum;
>>   }
>>
>>   /**
>> --
>> 2.17.1
> 
> @Emil, thank you for thoroughly reviewing the previous versions.
> 
> If your test succeeds and you are satisfied with the patch, remember to reply with a "Tested-by" tag for patchwork.
>