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

[annie li <annie.li@xxxxxxxxxx>]

On 2013-3-19 17:28, Paul Durrant wrote:
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> > bounces@xxxxxxxxxxxxx] On Behalf Of annie li
> > Sent: 19 March 2013 02:39
> > To: Ian Campbell
> > Cc: netdev@xxxxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx; Wei Liu; xen-
> > devel@xxxxxxxxxxxxx
> > 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.
> The flag is there to denote the existence of an 'extra' segment in the packet. 
> The 'extra' segment goes after the 1st segment and specifies metadata such as 
> the GSO type (TCPv4 is the only one at the moment but we'll need TCPv6 very 
> shortly) and the MSS.
For TCPv4 GSO, it seems one extra info request(NETTXF_extra_info) is 
enough in my winpv driver, and I did not process the XEN_NETIF_EXTRA_MORE.
Do you create two extra info requests for bothTCPv4 and TCPv6 GSO like 
following?
 * [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)


> Extra segments are certainly not redundant; the Citrix Windows PV drivers send 
> TSOs using them and handle LRO using them too.
About the LRO, upstream netback does not create any response with 
XEN_NETIF_EXTRA_MORE, so I assume your dom0 did such process?
Thanks
Annie

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