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Re: [Xen-devel] Re-reading domain configs on domain restart

On Mon, 2012-03-05 at 06:06 -0500, George Dunlap wrote:
> On Fri, Mar 2, 2012 at 10:15 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Fri, 2012-03-02 at 10:04 +0000, George Dunlap wrote:
> >> On Thu, Feb 23, 2012 at 3:16 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
> >> wrote:
> >> > On Thu, 2012-02-23 at 11:17 +0000, Ian Campbell wrote:
> >> >> As another (hopefully simple) idea how about a "xl dom-set-config" or
> >> >> similar which (only) updates the config stashed in the userinfo to be
> >> >> used on reboot? I would specifically exclude the possibility of this
> >> >> reconfiguring the running domain for simplicity.
> >> >
> >> > I've only lightly tested the following, but it seemed to do what I
> >> > expected (I used it to change memory from 512 to 1024 for a Windows VM
> >> > on reboot).
> >> >
> >> > I think there might be a better name, that better reflects the fact that
> >> > it doesn't actually change the config right now, any ideas?
> >> > "domain-config-override"?
> >> >
> >> > Thoughts on the general concept?
> >>
> >> Is the config used for anything other than a reboot?  If so, it might
> >> lead to unexpected behavior, if for example, I want to change config
> >> option X on reboot, but it's read (and acted on) before I get done
> >> with my various changes.  It would be good if the user could be sure
> >> that none of the changes would have any effect until the next reboot.
> >
> > The option takes a whole new configuration file so you are either using
> > the old or new configuration. There should be no intermediate state.
> >
> > AFAIK this file is used for any domain "restart", which includes reboot,
> > save+restore and migration.
> >
> > I think that any such changes which you might want to apply on reboot
> > you would also want to apply post migration or restore too.
> Wait, what kind of con fig changes would you want to apply across 
> save-restore?

The original request IIRC was to preserve changes made to a running
domain, such as adding a new VIF (using network-add) when the guest was
eventually restarted by updating the configuration used on restart. This
is useful if the restarts are out of your control (e.g. because the
customer does them himself). Today the VM will return to its original
state not the state right before the reboot. Especially annoying if this
results in loss of N/W connectivity for the user etc.

> Memory is something that can't be changed across save-restore, at
> least for an HVM domain.  Changes to CPU feature availability or
> hardware availability (e.g., NX bit, acpi, &c) will almost certainly
> cause the guest to crash;

Yes, you probably wouldn't want to be changing these using this
mechanism apart from on reboot.

>  I would think removing or changing devices
> (e.g., removing a NIC, or changing from one type to another) would
> have the same effect.

At least for PV devices this will be fine (modulo kernel bugs), a
migration looks pretty much like a complete teardown and rebuild of the
PV devices. It's also not a problem in the case of wanting to preserve
manually made changes on reboot which is the main motivation here.


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