Trigger code in domU from dom0

Trigger code in domU from dom0

[Asim]

Hi,

I want to trigger some code in my domU as soon as migration happens. I
can only know when to trigger in my destination dom0 but the code
should trigger in domU.
How do I go about doing this? Is there any script that completes or is
running that executes when a migration finishes or is about to finish?

Regards,
Asim

Re: Trigger code in domU from dom0

[Christopher Head]

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Hi,
I'm doing something very similar for a research project. Assuming your
domU is PV, look in linux/drivers/xen/core/reboot.c and
linux/drivers/xen/core/machine_reboot.c. You'll discover that the dom0
tools don't actually suspend the domU for migration: instead, they send
an event channel event to domU which then suspends itself via a
hypercall. All you have to do is pick a place sufficiently close to the
hypercall to run your own code.

Note that this is SMP safe as well: one of the first things the kernel
does when the suspend message is received is shut down all CPUs except
for #0, so only #0 will be alive around the hypercall. Combining that
with the lack of preemption in the Xen kernel, you have a situation
where your code is guaranteed to run before anything else.

Chris

Asim wrote:
| Hi,
|
| I want to trigger some code in my domU as soon as migration happens. I
| can only know when to trigger in my destination dom0 but the code
| should trigger in domU.
| How do I go about doing this? Is there any script that completes or is
| running that executes when a migration finishes or is about to finish?
|
| Regards,
| Asim
|
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Re: Trigger code in domU from dom0

[Christopher Head]

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As far as live migration, this should work perfectly as long as you want
your code to run **after** migration and not before. Live migration
differs from regular migration only in that the tools copy a pile of
memory pages without suspending the domain (meaning domU doesn't know
when this starts happening), then suspend the domain and copy the last
few pages. The event channel (or xenstore) mechanism used to trigger the
suspend is the same, though, meaning you can hook your code in and be
notified either between the mass copying and the copying of the last few
pages (before the hypercall), or after all migration is done (after the
hypercall).

I didn't see any documentation on the code, I just browsed the source.
For my purposes I'm not so much interested in IRQs, I'm more interested
in running before userspace gets a chance to run - so I'm not sure about
what you need for your project.

Chris

Asim wrote:
| Thanks a lot Chris for the response. I have some followup questions:-
|
| 1. Does this hold for "live" migration as well?
|
| 2. I want to run my code when all devices/irqs are available. The
| post_suspend looks like a good place to me. Did you get this
| information via mere source code browsing or there is a reference to
| it. It would be great for me to know where this is in migration stage
| in terms of dom0 - Is it after dom0 declared migration finished?
|
| 3. Thanks a lot for your help.
|
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Re: Trigger code in domU from dom0

[Asim]

Thanks a lot Chris -- I can see it working fine. I have placed my
calls after the hypercall in take_machine_down in
linux/drivers/xen/core/machine_reboot.c (if suspend is successful) and
it looks to be working good.

Thanks again.

Regards,
Asim

On 8/21/08, Christopher Head <chead@cs.ubc.ca> wrote:
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>
> As far as live migration, this should work perfectly as long as you want
> your code to run **after** migration and not before. Live migration
> differs from regular migration only in that the tools copy a pile of
> memory pages without suspending the domain (meaning domU doesn't know
> when this starts happening), then suspend the domain and copy the last
> few pages. The event channel (or xenstore) mechanism used to trigger the
> suspend is the same, though, meaning you can hook your code in and be
> notified either between the mass copying and the copying of the last few
> pages (before the hypercall), or after all migration is done (after the
> hypercall).
>
> I didn't see any documentation on the code, I just browsed the source.
> For my purposes I'm not so much interested in IRQs, I'm more interested
> in running before userspace gets a chance to run - so I'm not sure about
> what you need for your project.
>
> Chris
>
> Asim wrote:
> | Thanks a lot Chris for the response. I have some followup questions:-
> |
> | 1. Does this hold for "live" migration as well?
> |
> | 2. I want to run my code when all devices/irqs are available. The
> | post_suspend looks like a good place to me. Did you get this
> | information via mere source code browsing or there is a reference to
> | it. It would be great for me to know where this is in migration stage
> | in terms of dom0 - Is it after dom0 declared migration finished?
> |
> | 3. Thanks a lot for your help.
> |
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