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

From: Ian Campbell <ian.campbell@citrix.com>
To: Paul Durrant <Paul.Durrant@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Ian Jackson <Ian.Jackson@citrix.com>,
	"Keir (Xen.org)" <keir@xen.org>, Jan Beulich <jbeulich@suse.com>,
	"Tim (Xen.org)" <tim@xen.org>
Subject: Re: [PATCH v4 1/3] public/io/netif.h: document transmit and receive wire formats separately
Date: Fri, 8 Jan 2016 16:10:04 +0000	[thread overview]
Message-ID: <1452269404.26438.36.camel@citrix.com> (raw)
In-Reply-To: <e8f8ee97b87e40acb1fcabc9e28ec5a9@AMSPEX02CL03.citrite.net>

On Fri, 2016-01-08 at 15:56 +0000, Paul Durrant wrote:
> > -----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.

I think it would be worth saying that explicitly, even though your text is
strictly correct someone might not quite follow.

>  In fact the only Extra flag defined is the one that says there's another
> Extra :-)

As opposed to extra types of which there are several.

> 
> > 
> > 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.

Can you add a note please?

> > Same Q re which NETRXF_more_data is relevant.
> > 
> 
> Same answer :-)

Same request ;-)

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

Again perhaps worth a note.

> > 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.

Ta.

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