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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> Sent: Wednesday, April 28, 2021 2:22 PM
> 
> 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.

Please do.

> 
> > > +
> > > +/**
> > > + * 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?

You are right... this may be a question of personal preference - or application specific preference.

> 
> 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))  ?
> 

Good point. Double checking on Godbolt confirms the validity of your suggestion.

> 
> Regards,
> Olivier