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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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