Subject: Re: [RFC net-next PATCH 4/5] net: new XDP feature for reading HW rxhash from drivers

From: Jesper Dangaard Brouer <[email-masked]>
Date: Sun, 21 May 2017 18:04:35 +0200

On Sat, 20 May 2017 09:16:09 -0700
Tom Herbert <[email-masked]> wrote:

> > +/* XDP rxhash have an associated type, which is related to the RSS
> > + * (Receive Side Scaling) standard, but NIC HW have different mapping
> > + * and support. Thus, create mapping that is interesting for XDP.  XDP
> > + * would primarly want insight into L3 and L4 protocol info.
> > + *
> > + * TODO: Likely need to get extended with "L3_IPV6_EX" due RSS standard
> > + *
> > + * The HASH_TYPE will be returned from bpf helper as the top 32-bit of
> > + * the 64-bit rxhash (internally type stored in xdp_buff->flags).
> > + */
> > +#define XDP_HASH(x)            ((x) & ((1ULL << 32)-1))
> > +#define XDP_HASH_TYPE(x)       ((x) >> 32)
> > +
> > +#define XDP_HASH_TYPE_L3_SHIFT 0
> > +#define XDP_HASH_TYPE_L3_BITS  3
> > +#define XDP_HASH_TYPE_L3_MASK  ((1ULL << XDP_HASH_TYPE_L3_BITS)-1)
> > +#define XDP_HASH_TYPE_L3(x)    ((x) & XDP_HASH_TYPE_L3_MASK)
> > +enum {
> > +       XDP_HASH_TYPE_L3_IPV4 = 1,
> > +       XDP_HASH_TYPE_L3_IPV6,
> > +};
> > +
> > +#define XDP_HASH_TYPE_L4_SHIFT XDP_HASH_TYPE_L3_BITS
> > +#define XDP_HASH_TYPE_L4_BITS  5
> > +#define XDP_HASH_TYPE_L4_MASK                                          \
> > +       (((1ULL << XDP_HASH_TYPE_L4_BITS)-1) << XDP_HASH_TYPE_L4_SHIFT)
> > +#define XDP_HASH_TYPE_L4(x)    ((x) & XDP_HASH_TYPE_L4_MASK)
> > +enum {
> > +       _XDP_HASH_TYPE_L4_TCP = 1,
> > +       _XDP_HASH_TYPE_L4_UDP,
> > +};
> > +#define XDP_HASH_TYPE_L4_TCP (_XDP_HASH_TYPE_L4_TCP << XDP_HASH_TYPE_L4_SHIFT)
> > +#define XDP_HASH_TYPE_L4_UDP (_XDP_HASH_TYPE_L4_UDP << XDP_HASH_TYPE_L4_SHIFT)
> > +  
> Hi Jesper,
> 
> Why do we need these indicators for protocol specific hash? It seems
> like L4 and L3 is useful differentiation and protocol agnostic (I'm
> still holding out hope that SCTP will be deployed some day ;-) )

I'm not sure I understood the question fully, but let me try to answer
anyway.  To me it seems obvious that you would want to know the
protocol/L4 type, as this helps avoid hash collisions between UDP and
TCP flows.  I can easily imagine someone constructing an UDP packet
that could hash collide with a given TCP flow.

And yes, i40 support matching SCTP, and we will create a
XDP_HASH_TYPE_L4_SCTP when adding XDP rxhash support for that driver.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer