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    SubjectRe: [RFC] CPUID usage for interaction between Hypervisors and Linux.
    Jeremy Fitzhardinge wrote:
    > H. Peter Anvin wrote:
    >> And you're absolutely right that the guest may end up picking and
    >> choosing different parts of the interfaces. That's how it is supposed
    >> to work.
    > No, that would be a horrible, horrible mistake. There's no sane way to
    > implement that; it would mean that the hypervisor would have to have
    > some kind of state model that incorporates all the ABIs in a consistent
    > way. Any guest using multiple ABIs would effectively end up being
    > dependent on a particular hypervisor via a frankensteinian interface
    > that no other hypervisor would implement in the same way, even if they
    > claim to implement the same set of interfaces.
    > If the hypervisor just needs to deal with one at a time then it can have
    > relatively simple ABI<->internal state translation.
    > However, if you have the notion of hypervisor-agnostic or common
    > interfaces, then you can include those as part of the rest of the ABI
    > and make it sane (so Xen+common, hyperv+common, etc).

    It depends on what classes of interfaces you're talking about. I think
    you and Jun have a bit narrow definition of "ABI" in this context. This
    is functionally equivalent to hardware interfaces (after all, that is
    what the hypervisor ABI *is* as far as the kernel is concerned) -- noone
    expects, say, a SATA controller that can run in legacy IDE mode to also
    take AHCI commands at the same time, but the kernel *does* expect that a
    chipset which exports LAPIC, HPET, PMTMR and TSC clock sources can use
    all four at the same time. In the latter case the interfaces are
    inherently independent and refer to different chunks of hardware which
    just happen to be related in that they all are related to timing. In
    the former case, we're dealing with *one* piece of hardware which can
    operate in one of two modes.

    For hypervisors, you will end up with cases where you have both types --
    for example, KVM will happily use VMware's video interface, but that
    doesn't mean KVM wants to use VMware's interfaces for storage. This is
    exactly how it should be: the extent this kind of mix and match that is
    possible is a matter of the definition of the individual interfaces
    themselves, not of the overall architecture.


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