Re: [dpdk-dev] [PATCH 3/4] net: introduce functions to verify L4 checksums

[Olivier Matz <olivier.matz@6wind.com>]

Hi Morten,

Thank you for the review.

<...>

On Tue, Apr 27, 2021 at 05:07:04PM +0200, Morten Brørup wrote:
> > +static inline uint16_t
> > +rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const void
> > *l4_hdr)
> > +{
> > +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> > +
> > +	cksum = ~cksum;
> > +
> >  	/*
> > -	 * Per RFC 768:If the computed checksum is zero for UDP,
> > +	 * Per RFC 768: If the computed checksum is zero for UDP,
> >  	 * it is transmitted as all ones
> >  	 * (the equivalent in one's complement arithmetic).
> >  	 */
> >  	if (cksum == 0 && ipv4_hdr->next_proto_id == IPPROTO_UDP)
> >  		cksum = 0xffff;
> > 
> > -	return (uint16_t)cksum;
> > +	return cksum;
> > +}
> 
> The GCC static branch predictor treats the above comparison as likely. Playing around with Godbolt, I came up with this alternative:
> 
> 	if (likely(cksum != 0)) return cksum;
> 	if (ipv4_hdr->next_proto_id == IPPROTO_UDP) return 0xffff;
> 	return 0;

Good idea, this is indeed an unlikely branch.
However this code was already present before this patch,
so I suggest to add it as a specific optimization patch.

> > +
> > +/**
> > + * Validate the IPv4 UDP or TCP checksum.
> > + *
> > + * @param ipv4_hdr
> > + *   The pointer to the contiguous IPv4 header.
> > + * @param l4_hdr
> > + *   The pointer to the beginning of the L4 header.
> > + * @return
> > + *   Return 0 if the checksum is correct, else -1.
> > + */
> > +__rte_experimental
> > +static inline int
> > +rte_ipv4_udptcp_cksum_verify(const struct rte_ipv4_hdr *ipv4_hdr,
> > +			     const void *l4_hdr)
> > +{
> > +	uint16_t cksum = __rte_ipv4_udptcp_cksum(ipv4_hdr, l4_hdr);
> > +
> > +	if (cksum != 0xffff)
> > +		return -1;
> 
> The GCC static branch predictor treats the above comparison as likely, so I would prefer unlikely() around it.

For this one, I'm less convinced: should we decide here whether
the good or the bad checksum is more likely than the other?

Given it's a static inline function, wouldn't it be better to let
the application call it this way:
  if (likely(rte_ipv4_udptcp_cksum_verify(...) == 0))  ?


Regards,
Olivier