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

[Emil Berg <emil.berg@ericsson.com>]

> -----Original Message-----
> From: Mattias Rönnblom <hofors@lysator.liu.se>
> Sent: den 27 juni 2022 14:28
> To: Morten Brørup <mb@smartsharesystems.com>; Emil Berg
> <emil.berg@ericsson.com>; bruce.richardson@intel.com; dev@dpdk.org
> Cc: stephen@networkplumber.org; stable@dpdk.org; bugzilla@dpdk.org;
> olivier.matz@6wind.com
> Subject: Re: [PATCH v4] net: fix checksum with unaligned buffer
> 
> 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.
> 

Here:
https://www.agner.org/optimize/instruction_tables.pdf
it lists the latency of vmovdqa (aligned) as 6 cycles and the latency for vmovdqu (unaligned) as 7 cycles. So I guess there can be some difference.
Although in practice I'm not sure what difference it makes. I've not seen any difference in runtime between the two versions.

> >> +	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.
> >