Re: [Qemu-devel] [PATCH 2/10] Allow the monitor to be suspended during non-blocking op

[Avi Kivity <avi@qumranet.com>]

Daniel P. Berrange wrote:
>>
>> Or rather, such state should be part of the migration.  There's the 
>> question of whether to transform the path on the target, but "which 
>> media is in the drive" is part of the hardware state.
>>
>> (logically we would copy all of the data of all block devices, but 
>> that's not very practical, so we assume shared storage).
>>
>> What other commands are unsafe during migration? I exclude host device 
>> assignment which is obviously migration unfriendly.
>>     
>
> USB + virtio device attach/detach - well at least have the same need as
> media change - you'd need to propagate the change to the other side in
> some way. Currently migrate just assumes the management tool/admin has
> started QEMU on the destinations with the matching hardware config, and
> for libvirt to use the monitor to add USB /virtio devices at both ends
> has the race condition/synchronization problem .
>   

Any hotplug, for that matter.

The hardware topology should be part of the state; there is no other way 
to deal with hotplug.  Hotplug can create configurations that are 
impossible to recreate using the command line.

In general a hardware device has two parts: a guest visible part and a 
host interface part.  For networking, that's easily visible (-net nic 
and -net tap/user).  For block devices, the filename and caching mode is 
the host interface while, the interface type and index is the guest part.

Migration should transfer the guest part, and the host parts should be 
specified from the command line or monitor on the migration target, as 
they can change from host to host.

-- 
error compiling committee.c: too many arguments to function