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

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?

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

The only config option that comes to mind one might wish to change is
something like moving from a dom0 qemu to a stubdom; but that seems
like a very peculiar use-case.

> I think the right long term solution to this is
>        libxl_running_domain_to_config(ctx, domid, &libxl_domain_config)
> but that isn't going to happen for 4.2 at least and this is a good
> bandaid plus a useful generic facility.
>> I think the concept of having a "config-on-reboot" is a good one.  For
>> convenience, would it make sense to add a parameter to "xl reboot" to
>> take an argument (-c for example) that will do the config override
>> before issuing the reboot command?
> We have this for migrate and restore so I think extending that to reboot
> is a sensible idea.

Right -- what kinds of things were envisioned to change on a migrate?


> Ian.

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