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


From: Dor Laor
Subject: Re: [Qemu-devel] CPUID feature bits not saved with migration
Date: Thu, 16 Jul 2009 09:55:37 +0300
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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







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