linux-kernel - Re: [PATCH 1/7] PCI: Make sriov work with hotplug remove

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Date:	Tue, 24 Jan 2012 15:34:08 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Matthew Wilcox <matthew@....cx>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 1/7] PCI: Make sriov work with hotplug remove

On Mon, 2012-01-23 at 11:34 -0800, Linus Torvalds wrote:
> On Mon, Jan 23, 2012 at 10:45 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Why isn't this magically true in this case? If some *other* random
> > entry than the one that is being iterated over can magically be
> > removed, then the whole thing is just pure and utter crap, and no
> > amount of list maintenance can ever fix it.
> >
> > So explain.
> 
> Ahh. I finally understand what's going on. The virtual device attached
> to a physical device can go away, and it's on the same damn list.
> 
> That's broken. Virtual devices set up by a physical device should be
> *children* of the physical device, not "siblings". But that's
> apparently not what the broken virtual PCI layer does.

Thank the PCI SIG for that ... they are sibling functions (or even
devices in some case) of the PF :-(

> So I think that there are two possible solutions:
> 
>  (a) fix the virtual devices that are attached to physical devices to
> really be children of the physical device, with the physical device as
> a "bridge" to that virtual bus.

This will confuse various other aspects of the PCI code since they are
really siblings from an addressing standpoint (ie bus/dev/fn)

Cheers,
Ben.

> I think this is the correct solution from any sane standpoint (now the
> topology of the device tree actually matches the logical
> relationship), which is why I think this is the RightThing(tm) to do.
> And it should automatically fix this insane issue where removing a
> device from a bus can remove *multiple* devices from the same list.
> 
>  (b) if that isn't an option, and the virtual devices make a mess, at
> least don't make the code uglier, just do something like:
> 
>     while (!list_empty(&bus->devices)) {
>         struct pci_dev *dev = list_first_entry(struct pci_dev, bus_list);
> 
>         pci_stop_bus_device(dev);
>     }
> 
> which at least isn't butt ugly. Add a large comment about how
> pci_stop_bus_device() can remove multiple devices due to the virtual
> devices having been done badly.
> 
> Who is in charge of the whole 'is_virtfn' mess? How realistic is it to
> fix that crud?
> 
>                      Linus
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