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Re: [Xen-devel] XenStore as a data transfer path?

  • To: "'James Harper'" <james.harper@xxxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxx>
  • From: "Larry White" <lmw@xxxxxxxxxxx>
  • Date: Sat, 21 Jul 2012 10:04:02 -0500
  • Delivery-date: Sat, 21 Jul 2012 15:05:07 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>
  • Thread-index: Ac1m2/yuJJyO+ROQQxKMLV1mGoHskAABhbjAAABDEwAAAe6rkAAYxGOA

Hi James,
Thanks for your help with this.  I am brand new to Xen, so I appreciate the
information so I can get started on the correct path.

-----Original Message-----
From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx] 
Sent: Saturday, July 21, 2012 7:01 AM
To: Larry White; xen-devel@xxxxxxxxxxxxx
Subject: RE: [Xen-devel] XenStore as a data transfer path?

> >
> > Can the XenStore database be accessed by domU's?
> >
> >>Yes. That is how the frontend/backend devices set up their
> >>communication channels.
> Are you saying that the actual data going back and forth gets put into the
> XenStore?  I thought that the XenStore was mostly used for configuration
> keyword values and such that the underlying drivers needed.

>>Yes. It's how they set up their communication channels. The communication
>>channels are shared pages of memory and event channels, but they use
>>xenstore to set it up (eg so the backend can find the address of the
>>frontend ring.

I realize XenStore is used to help setup these channels.  However, for some
reason, I was thinking if this information is visible between both dom0 and
domU, I could use it to pass my data back and forth through.  Currently, my
domU Fedora Kernel is a plain vanilla version configured as a HVM which may
not be able to access the XenStore (not sure about this).  Do I need a
special guest kernel to perform Xen communication with dom0?  If so, what
about Windows?  Is there a special Guest Xen windows binary also for it?

I guess I was thinking that a simple XenStore only approach would eliminate
having to setup a split-driver model that utilized ring-buffers and such.
Of course, I can certainly do this if this is the correct thing to do.  I
just didn't think it had to be too complicated for such a small amount of
data that needs to be transferred.

> >
> > If so, could it be used to pass small amounts of data back and forth
> between
> > dom0 and domU's using XenStore key entries?  If this is possible, it
> > would seem that this could eliminate the need to create a "device"
> > driver for a simple communication path.
> >
> >>Yes, for certain values of "small". Can you say more about the sort of
> >>data you would be passing? In particular the throughput and latency
> >>requirements?
> I would say that the small amount of data here per transfer would be
> 32 bytes in length with a latency of around 500ms or so.

>>How fast though? Is it 32 bytes every 1ms, or 32 bytes every 20 seconds?

32 bytes every 500ms (1/2 second).


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