Re: [Xen-devel] [PATCH v3 2/9] xen-block: add document for mutli hardware queues/rings

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Oct 02, 2015 at 06:04:35PM +0200, Roger Pau Monné wrote:
> El 05/09/15 a les 14.39, Bob Liu ha escrit:
> > Document multi queues/rings of xen-block.
> > 
> > Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
> 
> As said by Konrad, you should send this against the Xen public headers
> also (or even before). I have a comment below.
> 
> > ---
> >  include/xen/interface/io/blkif.h |   32 ++++++++++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> > 
> > diff --git a/include/xen/interface/io/blkif.h 
> > b/include/xen/interface/io/blkif.h
> > index c33e1c4..b453b70 100644
> > --- a/include/xen/interface/io/blkif.h
> > +++ b/include/xen/interface/io/blkif.h
> > @@ -28,6 +28,38 @@ typedef uint16_t blkif_vdev_t;
> >  typedef uint64_t blkif_sector_t;
> >  
> >  /*
> > + * Multiple hardware queues/rings:
> > + * If supported, the backend will write the key "multi-queue-max-queues" to
> > + * the directory for that vbd, and set its value to the maximum supported
> > + * number of queues.
> > + * Frontends that are aware of this feature and wish to use it can write 
> > the
> > + * key "multi-queue-num-queues", set to the number they wish to use, which
> > + * must be greater than zero, and no more than the value reported by the 
> > backend
> > + * in "multi-queue-max-queues".
> > + *
> > + * For frontends requesting just one queue, the usual event-channel and
> > + * ring-ref keys are written as before, simplifying the backend processing
> > + * to avoid distinguishing between a frontend that doesn't understand the
> > + * multi-queue feature, and one that does, but requested only one queue.
> > + *
> > + * Frontends requesting two or more queues must not write the toplevel
> > + * event-channeland ring-ref keys, instead writing those keys under 
> > sub-keys
> > + * having the name "queue-N" where N is the integer ID of the queue/ring 
> > for
> > + * which those keys belong. Queues are indexed from zero.
> > + * For example, a frontend with two queues must write the following set of
> > + * queue-related keys:
> > + *
> > + * /local/domain/1/device/vbd/0/multi-queue-num-queues = "2"
> > + * /local/domain/1/device/vbd/0/queue-0 = ""
> > + * /local/domain/1/device/vbd/0/queue-0/ring-ref = "<ring-ref>"
> > + * /local/domain/1/device/vbd/0/queue-0/event-channel = "<evtchn>"
> > + * /local/domain/1/device/vbd/0/queue-1 = ""
> > + * /local/domain/1/device/vbd/0/queue-1/ring-ref = "<ring-ref>"
> > + * /local/domain/1/device/vbd/0/queue-1/event-channel = "<evtchn>"
> 
> AFAICT, it's impossible by design to use multiple queues together with
> multipage rings, is that right?
> 

As far as I can tell, these two features are not inherently coupled.
Whether you want to make (by design) them coupled together or not is
another matter. :-)

> Roger.
> 
> 
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