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

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Message-ID: <20081106183630.GD11773@parisc-linux.org>
Date:	Thu, 6 Nov 2008 11:36:30 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	"Fischer, Anna" <anna.fischer@...com>
Cc:	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


[Anna, can you fix your word-wrapping please?  Your lines appear to be
infinitely long which is most unpleasant to reply to]

On Thu, Nov 06, 2008 at 05:38:16PM +0000, Fischer, Anna wrote:
> > Where would the VF drivers have to be associated?  On the "pci_dev"
> > level or on a higher one?
> 
> A VF appears to the Linux OS as a standard (full, additional) PCI
> device. The driver is associated in the same way as for a normal PCI
> device. Ideally, you would use SR-IOV devices on a virtualized system,
> for example, using Xen. A VF can then be assigned to a guest domain as
> a full PCI device.

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).

> > Will all drivers that want to bind to a "VF" device need to be
> > rewritten?
> 
> Currently, any vendor providing a SR-IOV device needs to provide a PF
> driver and a VF driver that runs on their hardware. A VF driver does not
> necessarily need to know much about SR-IOV but just run on the presented
> PCI device. You might want to have a communication channel between PF
> and VF driver though, for various reasons, if such a channel is not
> provided in hardware.

That is one model.  Another model is to provide one driver that can
handle both PF and VF devices.  A third model is to provide, say, a
Windows VF driver and a Xen PF driver and only support Windows-on-Xen.
(This last would probably be an exercise in foot-shooting, but
nevertheless, I've heard it mooted).

> > Yeah, that's what I'm worried/curious about.  Without seeing the code
> > for such a driver, how can we properly evaluate if this infrastructure
> > is the correct one and the proper way to do all of this?
> 
> Yu's API allows a PF driver to register with the Linux PCI code and use
> it to activate VFs and allocate their resources. The PF driver needs to
> be modified to work with that API. While you can argue about how that API
> is supposed to look like, it is clear that such an API is required in some
> form. The PF driver needs to know when VFs are active as it might want to
> allocate further (device-specific) resources to VFs or initiate further
> (device-specific) configurations. While probably a lot of SR-IOV specific
> code has to be in the PF driver, there is also support required from
> the Linux PCI subsystem, which is to some extend provided by Yu's patches.

Everyone agrees that some support is necessary.  The question is exactly
what it looks like.  I must confess to not having reviewed this latest
patch series yet -- I'm a little burned out on patch review.

-- 
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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