linux-kernel - Re: [PATCH v10 4/7] PCI: add SR-IOV API for Physical Function driver

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Date:	Tue, 10 Mar 2009 09:37:53 +0800
From:	Yu Zhao <yu.zhao@...el.com>
To:	Greg KH <greg@...ah.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Randy.Dunlap" <rdunlap@...otime.net>
Subject: Re: [PATCH v10 4/7] PCI: add SR-IOV API for Physical Function driver

On Tue, Mar 10, 2009 at 03:39:01AM +0800, Greg KH wrote:
> On Mon, Mar 09, 2009 at 04:25:05PM +0800, Yu Zhao wrote:
> > > > +	pci_device_add(virtfn, virtfn->bus);
> > > 
> > > Greg is probably going to ding you here for adding the device, then
> > > creating the symlinks.  I believe it's now best practice to create the
> > > symlinks first, so there's no window where userspace can get confused.
> > 
> > Yes, but unfortunately we can't create links before adding a device.
> > I double checked device_add(), there is no place for those links to be
> > created before it sends uevent. So for now, we have to trigger another
> > uevent for those links.
> 
> What exactly are you trying to do with a symlink here that you need to
> do it this way?  I vaguely remember you mentioning this in the past, but
> I thought you had dropped the symlinks after our conversation about this
> very problem.

I'd like to create some symlinks to reflect the relationship between
Physical Function and its associated Virtual Functions. The Physical
Function is like a master device that controls the allocation of its
Virtual Functions and owns the device physical resource. The Virtual
Functions are like slave devices of the Physical Function. For example,
if 01:00.0 is a Physical Function and 02:00.0 is a Virtual Function
associated with 01:00.0. Then the symlinks (virtfnN and physfn) would
look like:

  $ ls -l /sys/bus/pci/devices/0000:01:00.0/
  ...
  ...  virtfn0 -> ../0000:02:00.0
  ...  virtfn1 -> ../0000:02:00.1
  ...  virtfn2 -> ../0000:02:00.2
  ...

  $ ls -l /sys/bus/pci/devices/0000:02:00.0/
  ...
  ... physfn -> ../0000:01:00.0
  ...

This is very useful for userspace applications, both KVM and Xen need
to know this kind of relationship so they can request the permission
from a Physical Function before using its associated Virtual Functions.

Thanks,
Yu
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