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