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

[Van De Ven, Arjan]

> > 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