linux-kernel - Re: [PATCH] pci: Save and restore VFs as a part of a reset

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Message-ID: <CAErSpo4htYwi3jfoxdj=W9VjVdd5gDjYcxg_i52VVz1sYQHL5A@mail.gmail.com>
Date:	Wed, 28 May 2014 14:14:35 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	Don Dutile <ddutile@...hat.com>
Subject: Re: [PATCH] pci: Save and restore VFs as a part of a reset

On Wed, May 28, 2014 at 10:39 AM, Alexander Duyck
<alexander.h.duyck@...el.com> wrote:
> On 05/27/2014 09:12 PM, Alex Williamson wrote:
>> On Tue, 2014-05-27 at 19:19 -0600, Bjorn Helgaas wrote:

>>> Maybe resetting the PF should just fail if there's an active VF.  If
>>> you need to reset the PF, you'd have to unbind the VFs first.
>>
>> The use case is certainly questionable, personally I'm not going to
>> expect VFs to continue working after the PF is reset.  Driver binding
>> gets complicated, especially when KVM doesn't actually bind devices to
>> use them.  Hopefully we'll get that out of the tree some day though.  I
>> suppose we could -EBUSY the PF reset as long as VFs are enabled.
>
> What I could do is go through and notify the VFs that they are about to
> get hit by a reset.  What they do with that information would be up to them.
>
> So if the VFs are loaded on the host I could then at least allow them to
> recover by saving and restoring the config space within the driver
> themselves.

I really like the idea of punting by failing the PF reset if there are
any active VFs.  That's a really easy way of making sure we aren't
going to blow up any guests.  What problems would it cause if we went
this route?

>>> This reminds me about an open problem: VFs can be on "virtual" buses,
>>> which aren't really connected in the hierarchy, and I don't think we
>>> have a nice way to iterate over them.  So probably pci_get_device() is
>>> the best we can do now.
>>
>> Yeah, those virtual buses don't have a bus->self, we just have to skip
>> to bus->parent->self.  pci_walk_bus() goes in the opposite direction,
>> but without an actual device hosting the bus, I don't see how it finds
>> it.  Thanks,
>
> It seems like we should be able to come up with something like
> pci_walk_vbus() though or something similar.  All we would need to do is
> search the VFs on the bus of the PF and all child busses to that bus if
> I am not mistaken.

I don't think that's going to work because the virtual buses don't
appear as the child bus of anything.

Bjorn
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