Re: [Qemu-devel] CPUID feature bits not saved with migration

[Dor Laor <dlaor@redhat.com>]

On 07/15/2009 06:12 PM, Jamie Lokier wrote:
> Anthony Liguori wrote:
>> It's unclear what to do about -cpu host.  If we did migrate cpuid
>> values, then -cpu would effectively be ignored after an incoming migration.

Actually, it might be better to build all of the guest configurations 
(devices, cpu, etc) from the migration stream, without stating them in 
the destination command line.

The destination command line should only contain host configuration.
It will bypass any type of conflict between the src/dst pairs.

>
> The new host might not support all the cpuid features of the old host,
> whether by -cpu host or explicit cpuid.  What happens then?
>
> For changing cpuid when migrating, as you might like to do with -cpu
> host for performance, is reboot-during-migrate useful?  It would make
> sure all disk state is committed to the image files asynchronously
> while the machine continues to run (just like normal migration), and
> at the last moment transfers control and the machine sees a reboot,
> permitting devices changes including cpuid change.
>
> CPU hotplug could be used for cpuid change in theory, but I doubt if
> any guests or guest apps would handle it well.

We should just fail migration in case the destination host cannot match 
the source requirements (cpuid, cpu vendor type (optionally emulate 
them), etc).
The complexity does not worth the gain and it won't be common practice.



>
> -- Jamie
>
>