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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Vrabel
> Sent: 02 December 2013 12:00
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; Wei Liu; Ian Campbell
> Subject: Re: [PATCH net-next] xen-netback: improve guest-receive-side flow
> control
> 
> 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@xxxxxxxxxxxxx; 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.
> 

TBH once I realised the value returned was different from xenvif_gop_skb() I 
didn' t look further; this function has needed putting out of its misery for a 
long time.

  Paul

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