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

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

Anthony Liguori wrote:
> Andre Przywara wrote:
>> Hi,
>>
>> while testing the guest multi-core code, I realized that all of the 
>> cpuid_* values (like cpuid_ext_features,...) from CPUX86State are not 
>> dumped in the migration stream. Is this intentional behavior (to let 
>> the user specify other bits in the target?) or has this just been 
>> forgotten?
>> Does KVM call the SET_CPUID2 ioctl after or before migration?
> 
> The cpuid bits are specified by the user.  They aren't changable by the 
> guest which is why they haven't been migrated.
OK, I see your point. What has happened with the idea of using a 
separate savevm section to transfer the guest configuration parts?

> 
> 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.
You do not want to migrate with -cpu host, this is only for a desktop 
virtualization use-case where you are pretty sure that you will not 
migrate. I have started working on "-cpu migrate", which uses some kind 
of least common denominator bits for the KVM case (that is all CPUs with 
hardware virtualization support). Currently there seems to be not much 
difference to qemu64, so maybe we don't need it.

Regards,
Andre.

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