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

[Andre Przywara]

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.
>
> The new host might not support all the cpuid features of the old host,
> whether by -cpu host or explicit cpuid.  What happens then?

If you plan to migrate, you should think of this in advance. I have a 
rough version of a tool to compute the least common denominator CPUID 
bits given either processor (code)names or host names. In the latter 
case it will login into the box and query the host's CPUID. The tool 
then generates a QEMU command line (like -cpu qemu64,-mwait,-popcnt) 
with which the guest should be started.
This should ensure that the guest always sees the same subset of the 
CPUs capabilities.

> 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.
Is that really useful? After all the sexy part of live migration is the 
"live" component...

> CPU hotplug could be used for cpuid change in theory, but I doubt if
> any guests or guest apps would handle it well.
Hotplugging could work for secondary processors, but hotplugging the BSP 
is kind of tricky. And this does not solve the userspace issues, where 
libraries detect CPU capabilities during startup and use optimized code 
paths. AFAIK there is no mechanism of informing those libraries about a 
CPUID change.

Regards,
Andre.

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