Re: [PATCH 5/6] net: add smaller IPv4 cksum function for simple cases

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 17 Oct 2024 20:03:13 +0100
Bruce Richardson <bruce.richardson@intel.com> wrote:

> On Thu, Oct 17, 2024 at 07:15:10PM +0200, Morten Brørup wrote:
> > > +/**
> > > + * Process the IPv4 checksum of an IPv4 header without any extensions.
> > > + *
> > > + * The checksum field does NOT have to be set by the caller, the field
> > > + * is skipped by the calculation.
> > > + *
> > > + * @param ipv4_hdr
> > > + *   The pointer to the contiguous IPv4 header.
> > > + * @return
> > > + *   The complemented checksum to set in the IP packet.
> > > + */
> > > +__rte_experimental
> > > +static inline uint16_t
> > > +rte_ipv4_cksum_simple(const struct rte_ipv4_hdr *ipv4_hdr)
> > > +{
> > > +	const uint16_t *v16_h;
> > > +	uint32_t ip_cksum;
> > > +
> > > +	/*
> > > +	 * Compute the sum of successive 16-bit words of the IPv4 header,
> > > +	 * skipping the checksum field of the header.
> > > +	 */
> > > +	v16_h = (const unaligned_uint16_t *)&ipv4_hdr->version_ihl;
> > > +	ip_cksum = v16_h[0] + v16_h[1] + v16_h[2] + v16_h[3] +
> > > +		v16_h[4] + v16_h[6] + v16_h[7] + v16_h[8] + v16_h[9];
> > > +
> > > +	/* reduce 32 bit checksum to 16 bits and complement it */
> > > +	ip_cksum = (ip_cksum & 0xffff) + (ip_cksum >> 16);
> > > +	ip_cksum = (ip_cksum & 0xffff) + (ip_cksum >> 16);
> > > +	ip_cksum = (~ip_cksum) & 0x0000FFFF;
> > > +	return (ip_cksum == 0) ? 0xFFFF : (uint16_t) ip_cksum;  
> > 
> > The zero exception does not apply to the checksum stored in the IP header, only to the checksum in the UDP header.
> >   
> 
> I was wondering about that, because I didn't see it mentioned anywhere in
> the RFCs I consulted, but on the other hand all the implementations in the
> code seemed to have the check for zero.
> 
> > > +}  
> > 
> > Besides that, for the series,  
> 
> So, just to confirm, the zero check at the end of the new ip_cksum_simple
> function should be removed and we always return the computed value
> directly?

Depends on usage.
  - if the computed value is zero, then 0xffff should be placed in
    the IP header.
  - often code use ip checksum code to see if incoming checksum is good.
    in that case zero means the checksum is valid.