Re: [PATCH net-next] xen-netback: improve guest-receive-side flow control

[David Vrabel <david.vrabel@citrix.com>]

On 02/12/13 11:55, Paul Durrant wrote:
>> -----Original Message-----
>> From: David Vrabel
>> Sent: 02 December 2013 11:52
>> To: Paul Durrant
>> Cc: xen-devel@lists.xen.org; Wei Liu; Ian Campbell
>> Subject: Re: [PATCH net-next] xen-netback: improve guest-receive-side flow
>> control
>>
>> On 28/11/13 13:11, Paul Durrant wrote:
>>> The flow control code relies on a double pass of tke skb, firstly to count
>>> the number of ring slots needed to supply sufficient grant references, and
>>> another to actually consume those references and build the grant copy
>>> operations. It transpires that, in some scenarios, the initial count and the
>>> number of references consumed differs and, after this happens a number
>> of
>>> times, flow control is completely broken.
>>
>> Can you describe in more detail the cases where it gets it wrong?
>>
> 
> I've already elaborated on this thread (at Annie Li's request). That
> explanation is probably a little long for inclusion in the comment though.

I understand that count_slot() going wrong is bad. What specifically is
it counting incorrectly.  What frag layout is causing it? If this is
understood there may be a more more minimal patch we can mark for stable.

David