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

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Date:	Thu, 6 Nov 2008 15:58:54 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	"Fischer, Anna" <anna.fischer@...com>, Greg KH <greg@...ah.com>,
	H L <swdevyid@...oo.com>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"grundler@...isc-linux.org" <grundler@...isc-linux.org>,
	"Chiang, Alexander" <achiang@...com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"rdreier@...co.com" <rdreier@...co.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [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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