netdev - Re: [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV

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Date:	Thu, 1 May 2014 16:16:51 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Sander Eikelenboom <linux@...elenboom.it>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	"David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<xen-devel@...ts.xen.org>
Subject: Re: [3.15-rc3] Bisected: xen-netback mangles packets between two
 guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY
 instead of copy" series.

On 01/05/14 15:05, Sander Eikelenboom wrote:
>
> Thursday, May 1, 2014, 3:49:45 PM, you wrote:
>
>> On 30/04/14 11:45, Sander Eikelenboom wrote:
>>>       Another point would be: what *correctness* testing is actually done on the xen-net* patches ?
>> I can speak only about my patches: I have manually tested them for the
>> usecases where they likely to make a difference, plus they went through
>> Xenserver's full test suite several times.
>
> I think Paul's patches for 3.14 also went through this testsuite fine, however
> it did have a bug in it. Does this testsuite include a test which causes a
> diverse pattern of frags (for both tx and rx case) ?
Unfortunately these tests doesn't directly try with various skb layouts, 
but it depends on the sending application/kernel what kind of packet 
they feed in to netback/netfront.
I was always thinking we should create a testing facility where we can 
generate various different skb's and feed them in at an arbitrary part 
of the networking stack. Or does such thing already exist?
>
>
>>>       As i suspect this is again about fragmented packets .. that doesn't seem to be included in any test case while it actually seems to be a case which is hard to get right...
>> Beware, there are frags and frag_list which are two entirely different
>> things with confusing names. In netback case, frags are used to pass
>> through large packets for a long time. frag_list is used only since my
>> grant mapping patches, to handle older guests (see comment in
>> include/xen/interface/io/netif.h for XEN_NETIF_NR_SLOTS_MIN)
>
> Ah ok .. it's not about the frags in the packets being handled, but the frag
> mechanism is supposed to be used internally ?
Yes, the skb on the frag_list should contain no linear data but that 
extra frag the guest sent to netback. After the grant operations are 
done, xenvif_handle_frag_list coalesce the frags and that extra skb into 
brand new, PAGE_SIZE frags.

>
> If so .. there is at least something wrong in the "older guest" detection,
> because both dom0 and PV guests are running the same 3.15-rc3 kernel.
That seems very odd ... Can you check ethtool -S vifX.Y in Dom0? 
tx_frag_overflow will count the packets with too many frags
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