Re: [Qemu-devel] virtio scsi host draft specification, v3

[Hannes Reinecke]

On 06/12/2011 09:51 AM, Michael S. Tsirkin wrote:
> On Fri, Jun 10, 2011 at 02:55:35PM +0200, Hannes Reinecke wrote:
> > > Device operation: request queues
> > > --------------------------------
> > >
> > > The driver queues requests to an arbitrary request queue, and they are
> > > used by the device on that same queue.
> > What about request ordering?
> > If requests are placed on arbitrary queues you'll inevitably run on
> > locking issues to ensure strict request ordering.
> > I would add here:
> >
> > If a device uses more than one queue it is the responsibility of the
> > device to ensure strict request ordering.
>
> Maybe I misunderstand - how can this be the responsibility of
> the device if the device does not get the information about
> the original ordering of the requests?
>
> For example, if the driver is crazy enough to put
> all write requests on one queue and all barriers
> on another one, how is the device supposed to ensure
> ordering?
Which is exactly the problem I was referring to.
When using more than one channel the request ordering
_as seen by the initiator_ has to be preserved.

This is quite hard to do from a device's perspective;
it might be able to process the requests _in the order_ they've 
arrived, but it won't be able to figure out the latency of each 
request, ie how it'll take the request to be delivered to the initiator.

What we need to do here is to ensure that virtio will deliver
the requests in-order across all virtqueues. Not sure whether it 
does this already.

Cheers,

Hannes
--
Dr. Hannes Reinecke                   zSeries & Storage
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