Re: [Xen-devel] Guest to Host communication

["Jose A. Lopes" <jabolopes@xxxxxxxxxx>]

On Thu, Oct 31, 2013 at 02:21:38PM +0000, George Dunlap wrote:
> On Tue, Oct 22, 2013 at 2:42 PM, Jose A. Lopes <jabolopes@xxxxxxxxxx> wrote:
> > On Tue, Oct 22, 2013 at 01:18:56PM +0000, Paul Durrant wrote:
> >> > -----Original Message-----
> >> > From: Jose A. Lopes [mailto:jabolopes@xxxxxxxxxx]
> >> > Sent: 22 October 2013 13:49
> >> > To: Paul Durrant
> >> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> > Subject: Re: [Xen-devel] Guest to Host communication
> >> >
> >> > Hi,
> >> >
> >> > If I understood correctly, Xenstore requires writing a driver and
> >> > loading it inside the VM.  Is this correct?
> >> >
> >>
> >> Well, yes you'll need some code in the guest, but drivers already exist 
> >> for linux and windows so you could just use them.
> >>
> >> > If so, then it seems this approach would require writing different
> >> > drivers for different OSes, such as, Linux and Windows.  Currently, we
> >> > are just exploring different design options and we would like to aim
> >> > at a uniform approach across different OSes. Is there an option like 
> >> > that ?
> >> >
> >>
> >> What option do you expect that doesn't involve writing at least some code 
> >> for each OS you want to use?
> >
> > We were thinking of simply attaching a device.
> > For example, add a SSD disk in DomU which is backed in Dom0 by a file.
> > Is this possible?
> 
> Of course you can attach as many disks as you want; or you could just
> have dom0 look for a file in the primary filesystem.  But whether you
> use the primary filesystem or a secondary one, you still have the
> basic problem of introspection: if this is going to be a filesystem,
> then there may be a difference at any given time between what domU
> thinks the state of the filesystem is, and what dom0 can see on the
> disk (because there may be things in domU's buffer cache which haven't
> "hit the disk" yet).

We have changed our use case to the situation where the instance
writes a few bytes to this device.  However, Ganeti only needs this
information once the instance has stopped.  Therefore, while the
instance is running it can write and overwrite the information because
it is not important.

I wonder if it is possible to change the caching policy of the device
in order to immediately flush the writes.  This is not so important
because I imagine that by the time the instance has stopped (if it
didn't crash), the cache should have been flushed.

We were however thinking about a usb serial device to make it easier
to do successive writes within the instance.

Jose

> 
> I suppose one option would be to make a block device with "magic" --
> i.e., "write to block 0 for this bit of info, block 2 for this bit".
> But the way that you write specific blocks is different for each OS
> anyway -- you're essentially writing a driver for this "magic block
> device" anyway; *and* you have to write new code in dom0 to look at
> this device and notice changes to it, which will be non-trivial.
> 
> Fortunately, there already is a well-defined way of doing this which
> already has drivers and libraries for all major operating systems
> (Linux, Windows, NetBSD, FreeBSD, probably even Gnu Hurd): xenstore.
> It already has "watch" functionality, so that a program in dom0 can
> say, "Tell me when this key {....} is created or changed" and then
> contentedly go to sleep, secure in the knowledge that the well-tested
> data path will wake it up when the guest writes to it.  It's almost
> certainly your best solution.
> 
>  -George

-- 
Jose Antonio Lopes
Ganeti Engineering
Google Germany GmbH
Dienerstr. 12, 80331, München

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