Re: [Xen-devel] Guest to Host communication

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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).

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

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel