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

[Paolo Bonzini]

On 20/02/2018 01:00, Van De Ven, Arjan wrote:
>>> the guest is not the problem; guests obviously will already honor
>>> if Enhanced IBRS is enumerated. The problem is mixed migration
>>> pools where the hypervisor
>>> may need to decide to not pass this enumeration through to the
>>> guest.
>> 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.

Paolo