linux-kernel - Re: [PATCH 6/6 v3] PCI: document the change

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Date:	Mon, 13 Oct 2008 19:08:27 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	"Dong, Eddie" <eddie.dong@...el.com>
Cc:	"Zhao, Yu" <yu.zhao@...el.com>, linux-pci@...r.kernel.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Grant Grundler <grundler@...isc-linux.org>,
	Alex Chiang <achiang@...com>,
	Roland Dreier <rdreier@...co.com>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 6/6 v3] PCI: document the change

On Tue, Oct 14, 2008 at 08:23:34AM +0800, Dong, Eddie wrote:
> Matthew Wilcox wrote:
> > Wouldn't it be more useful to have the iov/N directories
> > be a symlink to the actual pci_dev used by the virtual
> > function? 
> 
> The main concern here is that a VF may be disabed such as when PF enter
> D3 state or undergo an reset and thus be plug-off, but user won't
> re-configure the VF in case the PF return back to working state.

If we're relying on the user to reconfigure virtual functions on return
to D0 from D3, that's a very fragile system.

> >> +For network device, there are:
> >> +       - /sys/bus/pci/devices/BB:DD.F/iov/N/mac
> >> +       - /sys/bus/pci/devices/BB:DD.F/iov/N/vlan
> >> +               (value update will notify PF driver)
> > 
> > We already have tools to set the MAC and VLAN parameters
> > for network devices.
> 
> Do you mean Ethtool? If yes, it is impossible for SR-IOV since the
> configuration has to be done in PF side, rather than VF.

I don't think ethtool has that ability; ip(8) can set mac addresses and
vconfig(8) sets vlan parameters.

The device driver already has to be aware of SR-IOV.  If it's going to
support the standard tools (and it damn well ought to), then it should
call the PF driver to set these kinds of parameters.

> > I'm not 100% convinced about this API.  The assumption
> > here is that the driver will do it, but I think it should
> > probably be in the core.  The driver probably wants to be
> 
> Our concern is that the PF driver may put an default state when it is
> loaded so that SR-IOV can work without any user level configuration, but
> of course the driver won't dynamically change it.
> Do u mean we remove this ability?
> 
> > notified that the PCI core is going to create a virtual
> > function, and would it please prepare to do so, but I'm
> > not convinced this should be triggered by the driver. 
> > How would the driver decide to create a new virtual
> > function?  

Let me try to explain this a bit better.

The user decides they want a new ethernet virtual function.  In the
scheme as you have set up:

1. User communicates to ethernet driver "I want a new VF"
2. Ethernet driver tells PCI core "create new VF".

I propose:

1. User tells PCI core "I want a new VF on PCI device 0000:01:03.0"
2. PCI core tells driver "User wants a new VF"

My scheme gives us a unified way of creating new VFs, yours requires each
driver to invent a way for the user to tell them to create a new VF.
Unless I've misunderstood your code and docs.

> > From my reading of the SR-IOV spec, this isn't how it's
> > supposed to work.  The device is supposed to be a fully
> > functional PCI device that on demand can start peeling
> > off virtual functions; it's not supposed to boot up and
> > initialise all its virtual functions at once. 
> 
> The spec defines either we enable all VFs or Disable. Per VF enabling is
> not supported.
> Is this what you concern?

I don't think that's true.  The spec requires you to enable all the
VFs from 0 to NumVFs, but NumVFs can be lower than TotalVFs.  At least,
that's how I read it.

But no, that isn't my concern.  My concern is that you've written a
driver here that seems to be a stub driver.  That doesn't seem to be
how SR-IOV is supposed to work; it's supposed to be a fully-functional
driver that has SR-IOV knowledge added to it.

-- 
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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