Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 12 March 2014 15:43
> To: Paul Durrant
> Cc: Zoltan Kiss; Ian Campbell; xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu; Tim
> (Xen.org)
> Subject: Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness
> 
> On Wed, Mar 12, 2014 at 03:23:09PM +0000, Paul Durrant wrote:
> [...]
> > > > Actually ancient memory tells me that, unfortunately, netback's
> backend-
> > > >frontend GSO protocol is broken in this way... it requires one more
> > > response slot than the number of requests it consumes (for the extra
> > > metadata), which means that if the frontend keeps the ring full you can
> get
> > > overflow. It's a bit of a tangent though, because that code doesn't use
> this
> > > macro (or in fact check the ring has space in any way IIRC). The prefix
> variant
> > > of the protocol is ok though.
> > >
> > > I think it's not: it consumes a request for the metadata, and when the
> > > packet is grant copied to the guest, it creates a response for that slot
> > > as well.
> >
> > As explained verbally, it doesn't consume a request for the 'extra' info. 
> > Let
> me elaborate here for the benefit of the list...
> >
> > In xenvif_gop_skb(), in the non-prefix GSO case, a single request is
> consumed for the header along with a meta slot which is used to hold the
> GSO data. Later on in xenvif_rx_action() the code calls make_rx_response()
> for the header, but then *before* moving onto the next meta slot it makes
> an 'extra' response for the GSO metadata. So - one meta slot - one request
> consumed, but two responses produced.
> > So this mechanism totally relies on the netfront driver not completely 
> > filling
> the shared ring. If it ever does, you'll get overflow.
> >
> 
> (... which reminds me of the heisenbug Sander is seeing.)
> 
> But do we not check for there's enough space in the ring before
> procceeding?
> 

Apparently not, but TBH I cannot figure out how on earth this ever worked at 
all. If netback is genuinely issuing 2 reponses for 1 consumed request then it 
must trash an unconsumed request at some point - but from my reading of the 
code that really does seem to be what it's doing.

  Paul

> Wei.
> 
> >   Paul
> >
> >

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