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

[Andre Przywara <andre.przywara@amd.com>]

Dor Laor wrote:
> 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.
I think there were some ideas on implementing this (transfer of guest 
configuration in a separate savevm section) in the past. Has someone 
started looking at this?

>>
>> 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.

For items not efficiently emulate-able (like SSE4) I agree. But I would 
try to make this list as short as possible. For instance I have an idea 
on how to cope with missing host's NX/XD capability, so one does not 
need to dump this feature because there is a single incapable box in the 
migration pool.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
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