netdev - RE: [Xen-devel] [PATCH 1/4] xen-netfront: remove unused variable `extra'

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Date:	Tue, 19 Mar 2013 03:02:48 +0000
From:	James Harper <james.harper@...digoit.com.au>
To:	annie li <annie.li@...cle.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: RE: [Xen-devel] [PATCH 1/4] xen-netfront: remove unused variable
	`extra'

> 
> On 2013-3-18 20:14, Ian Campbell wrote:
> > On Mon, 2013-03-18 at 12:04 +0000, Wei Liu wrote:
> >> On Mon, 2013-03-18 at 11:42 +0000, Ian Campbell wrote:
> >>> On Mon, 2013-03-18 at 10:35 +0000, Wei Liu wrote:
> >>>
> >>> I think a few more words are needed here since from the code you are
> >>> removing it seems very much like gso is used for something. If you have
> >>> a proof that the "extra = gso" case is never hit then please explain it.
> >>> Perhaps a reference to the removal of the last user?
> >>>
> >>> Or maybe it is the case that it should be used and the bug is that it
> >>> isn't?
> >>>
> >> Looks like the latter one. 'extra' field should  be used to get hold of
> >> the last extra info in the ring. ;-)
> >>
> >> But, the only extra info in upstream kernel is
> XEN_NETIF_EXTRA_TYPE_GSO,
> >> so there's really no other extra info in the ring at that point. Could
> >> it be possible that it is something from classic Xen kernel?
> > The classic kernel netfront has exactly the same code it seems and
> > netif_extra_type_gso is the only one I've ever heard of.
> >
> > Maybe this extra thing is just redundant unless/until a second extra
> > comes along.
> 
> In our windows pv driver, we do not process this for GSO in tx path
> either. Maybe we ignored processing for some special GSO?
> 
> BTW, what is XEN_NETIF_EXTRA_FLAG_MORE actually for? Backend only
> processes it in xen_netback_tx_build_gops, but netfront xmit path does
> not really set this flag. I did process it in rx path of my windows pv
> driver(linux netfront did that too), but it seems unnecessary since
> netback does not set this flag at all.
> 

This flag is set to say if there is another 'extra' ring entry. From netif.h:

/*
 * This is the 'wire' format for packets:
 *  Request 1: netif_tx_request -- NETTXF_* (any flags)
 * [Request 2: netif_tx_extra]  (only if request 1 has NETTXF_extra_info)
 * [Request 3: netif_tx_extra]  (only if request 2 has XEN_NETIF_EXTRA_MORE)
 *  Request 4: netif_tx_request -- NETTXF_more_data
 *  Request 5: netif_tx_request -- NETTXF_more_data
 *  ...
 *  Request N: netif_tx_request -- 0
 */

I think the only extra type is GSO so you'll probably never see it, but that's what it's for.

James
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