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

From: annie li <[email-masked]>
Date: Tue, 19 Mar 2013 10:39:14 +0800

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.

Thanks
Annie