[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On 12/03/14 11:38, Paul Durrant wrote:
-----Original Message-----
From: Paul Durrant
Sent: 12 March 2014 11:26
To: Ian Campbell; Zoltan Kiss
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu; Tim (Xen.org)
Subject: RE: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
bounces@xxxxxxxxxxxxx] On Behalf Of Ian Campbell
Sent: 12 March 2014 10:28
To: Zoltan Kiss
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu; Tim (Xen.org)
Subject: Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

On Tue, 2014-03-11 at 23:24 +0000, Zoltan Kiss wrote:
On 11/03/14 15:44, Ian Campbell wrote:

Is it the case that this macro considers a request to be unconsumed if
the *response* to a request is outstanding as well as if the request
itself is still on the ring?
I don't think that would make sense. I think everywhere where this
is called the caller is not interested in pending request (pending means
consumed but not responded)

It might be interested in such pending requests in some of the
pathological cases I allude to in the next paragraph though?

For example if the ring has unconsumed requests but there are no slots
free for a response, it would be better to treat it as no unconsumed
requests until space opens up for a response, otherwise something else
just has to abort the processing of the request when it notices the lack
of space.

Huh? The act of consuming the request will create the space for the
response. If responses and requests are not balanced then I'd argue that the
protocol is broken.

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.

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.