Re: [PATCH 0/16 v6] PCI: Linux kernel SR-IOV support

[Matthew Wilcox <matthew@wil.cx>]

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."