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Re: [Xen-devel] shutdown question - PV vs HVM

To: Russ Blaine <russell.blaine@xxxxxxx>
Subject: Re: [Xen-devel] shutdown question - PV vs HVM
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 03 Sep 2008 20:02:36 +0100
Cc: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] shutdown question - PV vs HVM
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On 3/9/08 19:01, "Russ Blaine" <russell.blaine@xxxxxxx> wrote:

>> For a PV guest, a shutdown node is written into xenstore which the guest
>> picks up. The guest will then usually initiate a clean shutdown.
>> The same happens for an HVM guest with PV drivers installed. If an HVM guest
>> has no PV drivers (And so will obviously not be watching xenstore) then xend
>> notifies the shutdown immediately to Xen. This causes Xen to immediately
>> transition the guest to shutdown state, and no longer execute the guest,
>> with no chance for clean shutdown. If there are no PV drivers installed,
>> clean shutdown needs to be initiated from within the guest.
> Why are HVM domains treated differently for shutdown? Guest OSes already know
> how to do a clean ACPI-initiated shutdown, so why not use that for shutdown in
> the presence of PV drivers?
> The current method requires extra complexity in PV drivers. I imagine that it
> either fell out of porting PV drivers from PV domains to HVM domains or is
> just 
>   paranoia about losing data during ACPI shutdown.

Really it just fell out of porting the PV drivers. The shutdown node must be
watched as it is part of the save/restore/migrate protocol. Hooking shutdown
off the same node is trivial within the PV drivers.

Another issue is how to reliably trigger an S5 transition (poweroff) in a
wide range of OS versions. It might be hooked off the 'lid switch'. OTOH it
may not be externally triggerable at all...

 -- Keir

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