Re: [PATCH 2/2] net: have checksum routines accept unaligned data

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Jul 08, 2022 at 01:52:12PM +0000, Mattias Rönnblom wrote:
> On 2022-07-08 15:02, Morten Brørup wrote:
> >> From: Mattias Rönnblom [mailto:mattias.ronnblom@ericsson.com]
> >> Sent: Friday, 8 July 2022 14.44
> >>
> >> On 2022-07-07 23:44, Morten Brørup wrote:
> >>>> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> >>>> Sent: Thursday, 7 July 2022 20.35
> >>>>
> >>>> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> >>>>
> >>>> __rte_raw_cksum() (used by rte_raw_cksum() among others) accessed
> >> its
> >>>> data through an uint16_t pointer, which allowed the compiler to
> >> assume
> >>>> the data was 16-bit aligned. This in turn would, with certain
> >>>> architectures and compiler flag combinations, result in code with
> >> SIMD
> >>>> load or store instructions with restrictions on data alignment.
> >>>>
> >>>> This patch keeps the old algorithm, but data is read using memcpy()
> >>>> instead of direct pointer access, forcing the compiler to always
> >>>> generate code that handles unaligned input. The __may_alias__ GCC
> >>>> attribute is no longer needed.
> >>>>
> >>>> The data on which the Internet checksum functions operates are
> >> almost
> >>>> always 16-bit aligned, but there are exceptions. In particular, the
> >>>> PDCP protocol header may (literally) have an odd size.
> >>>>
> >>>> Performance impact seems to range from none to a very slight
> >>>> regression.
> >>>>
> >>>> Bugzilla ID: 1035
> >>>> Cc: stable@dpdk.org
> >>>>
> >>>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> >>>> ---
> >>>>    lib/net/rte_ip.h | 19 ++++++++++++-------
> >>>>    1 file changed, 12 insertions(+), 7 deletions(-)
> >>>>
> >>>> diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
> >>>> index b502481670..a9e6251f14 100644
> >>>> --- a/lib/net/rte_ip.h
> >>>> +++ b/lib/net/rte_ip.h
> >>>> @@ -160,18 +160,23 @@ rte_ipv4_hdr_len(const struct rte_ipv4_hdr
> >>>> *ipv4_hdr)
> >>>>    static inline uint32_t
> >>>>    __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 void *end;
> >>>
> >>> I would set "end" here instead, possibly making the pointer const
> >> too. And add spaces around '/'.
> >>> const void * const end = RTE_PTR_ADD(buf, (len / sizeof(uint16_t)) *
> >> sizeof(uint16_t));
> >>>
> >>
> >> I don't think that makes the code more readable.
> > 
> > It's only a matter of taste... Your code, your decision. :-)
> > 
> > I think the spaces are required by the coding standard; not sure, though.
> > 
> 
> If it isn't in the coding standard, it should be. But if you add spaces, 
> you have to break the line, to fit into 80 characters. A net loss, IMO.
> 

Just FYI, lines up to 100 chars are ok now. Automated checkpatch checks as
shown in patchwork only flag lines longer than that.

/Bruce