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

[Paolo Bonzini]

On 20/02/2018 15:59, Van De Ven, Arjan wrote:
> 
>>> 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.
> 
> that makes sense, compatibility in this direction can be done
> and is useful as you move a fleet of servers forward
> 
>> Backwards migration to older hypervisors also happens sometimes, but in
>> general it creates more userspace than kernel issues.
> 
> that direction is obviously harder

>From KVM's point of view it's mostly a matter of having a complete ioctl
API right.  I'm not that much worried by it.

Paolo