Re: Guest to Host communication

[George Dunlap <george.dunlap@eu.citrix.com>]

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@google.com> wrote:
>>> On Tue, Oct 22, 2013 at 01:18:56PM +0000, Paul Durrant wrote:
>>>>> -----Original Message-----
>>>>> From: Jose A. Lopes [mailto:jabolopes@google.com]
>>>>> Sent: 22 October 2013 13:49
>>>>> To: Paul Durrant
>>>>> Cc: xen-devel@lists.xenproject.org
>>>>> 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