Re: [PATCH v4 1/3] public/io/netif.h: document transmit and receive wire formats separately

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
[snip]
> >   * Guest transmit
> >   * ==============
> >   *
> > + * This is the 'wire' format for packets:
> > + *  Fragment 1: netif_tx_request_t  - flags = NETTXF_*
> > + *                                    size = total packet size
> > + * [Extra 1: netif_extra_info_t]    - (only if fragment 1 flags include
> > + *                                     NETTXF_extra_info)
> > + * [Extra N: netif_extra_info_t]    - (only if extra N-1 flags include
> > + *                                     XEN_NETIF_EXTRA_MORE)
> > + *  ...
> > + *  Fragment N: netif_tx_request_t  - (only if fragment N-1 flags include
> > + *                                     NETTXF_more_data)
> 
> For Fragment 2 is it the Flags of Fragment N-1 = 1 which are relevant, or
> the flags in the optional Extra N which may be in the middle (i.e. the
> immediately preceding slot)?

It's Fragment N-1's flags. The flags on the Extras are not relevant. In fact the only Extra flag defined is the one that says there's another Extra :-)

> 
> Am I correct in remembering that in the presence of NETTXF_more_data the
> only way to know the actual size of Fragment 1 is to take away the total of
> all the extras from Frag 1's size?

That's right.

> 
> > + *                                    flags = 0
> > + *                                    size = fragment size
> > + *
> >   * Ring slot size is 12 octets, however not all request/response
> >   * structs use the full size.
> >   *
> > @@ -202,6 +202,19 @@
> >   * Guest receive
> >   * =============
> >   *
> > + * This is the 'wire' format for packets:
> > + *  Fragment 1: netif_rx_request_t  - flags = NETRXF_*
> > + *                                    size = fragment size
> > + * [Extra 1: netif_extra_info_t]    - (only if fragment 1 flags include
> > + *                                     NETRXF_extra_info)
> > + * [Extra N: netif_extra_info_t]    - (only if extra N-1 flags include
> > + *                                     XEN_NETIF_EXTRA_MORE)
> > + *  ...
> > + *  Fragment N: netif_rx_request_t  - (only if fragment N-1 flags include
> > + *                                     NETRXF_more_data)
> > + *                                    flags = 0
> > + *                                    size = fragment size
> 
> Same Q re which NETRXF_more_data is relevant.
> 

Same answer :-)

> In this path there is no indication of the total packet size other than
> adding everything up?
> 

Correct. 

> Given that they differ in a subtle way would a quick but explicit "NOTE: RX
> and TX differ" be a useful addition do you think?
> 

Yes, that's probably a good plan. I'll stick an extra comment in to that effect.

  Paul

> > + *
> >   * Ring slot size is 8 octets.
> >   *
> >   * rx request (netif_rx_request_t)
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