Re: [Xen-devel] Guest to Host communication

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 31/10/13 16:08, Jose A. Lopes wrote:
> 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.
Oh, right -- in that case, probably the easiest thing to do would be to 
just create a file in the guest filesystem and have your tools look 
inside to see if it's there.  You could have a secondary disk just for 
that purpose, if that makes it easier to make it the same across instances.
 -George

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