Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs

[Paolo Bonzini]

On 20/02/2018 15:08, Van De Ven, Arjan wrote:
>>>> For bonus points:  What should happen to a VM that is live migrated
>>>> from one hypervisor to another, and the hypervisors have different
>>>> IBRS support?
>>>
>>> Doctor Doctor it hurts when I do this....
>>>
>>> Migration tends to only work between HV's that are relatively
>>> homogeneous, that's nothing new...
>>
>> No Arjan, this is just wrong.  Well, I suppose it's right in the present
>> tense with the IBRS mess on Skylake, but it's _not_ been true until last
>> year.
> 
> I meant software wise. You're not going to live migrate from xen to
> kvm or backwards. or between very radically different versions of the
> kvm stack.

Forwards migration to a radically newer version certainly happens.  So
when the source hypervisor was too old to tell the VM about IBRS_ALL,
for example, migration should work properly and the VM should perform
well on the destination hypervisor.

Backwards migration to older hypervisors also happens sometimes, but in
general it creates more userspace than kernel issues.

Paolo