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    SubjectRe: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support
    On Thu, Nov 06, 2008 at 04:38:40PM -0600, Anthony Liguori wrote:
    > >It's not clear thats the right solution. If the VF devices are _only_
    > >going to be used by the guest, then arguably, we don't want to create
    > >pci_devs for them in the host. (I think it _is_ the right answer, but I
    > >want to make it clear there's multiple opinions on this).
    >
    > The VFs shouldn't be limited to being used by the guest.
    >
    > SR-IOV is actually an incredibly painful thing. You need to have a VF
    > driver in the guest, do hardware pass through, have a PV driver stub in
    > the guest that's hypervisor specific (a VF is not usable on it's own),
    > have a device specific backend in the VMM, and if you want to do live
    > migration, have another PV driver in the guest that you can do teaming
    > with. Just a mess.

    Not to mention that you basically have to statically allocate them up
    front.

    > What we would rather do in KVM, is have the VFs appear in the host as
    > standard network devices. We would then like to back our existing PV
    > driver to this VF directly bypassing the host networking stack. A key
    > feature here is being able to fill the VF's receive queue with guest
    > memory instead of host kernel memory so that you can get zero-copy
    > receive traffic. This will perform just as well as doing passthrough
    > (at least) and avoid all that ugliness of dealing with SR-IOV in the guest.

    This argues for ignoring the SR-IOV mess completely. Just have the
    host driver expose multiple 'ethN' devices.

    > This eliminates all of the mess of various drivers in the guest and all
    > the associated baggage of doing hardware passthrough.
    >
    > So IMHO, having VFs be usable in the host is absolutely critical because
    > I think it's the only reasonable usage model.
    >
    > Regards,
    >
    > Anthony Liguori
    > --
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    --
    Matthew Wilcox Intel Open Source Technology Centre
    "Bill, look, we understand that you're interested in selling us this
    operating system, but compare it to ours. We can't possibly take such
    a retrograde step."


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