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

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