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

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Date:	Wed, 28 May 2014 16:34:04 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>,
	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>
Subject: Re: [PATCH] pci: Save and restore VFs as a part of a reset

On 05/28/2014 04:14 PM, Bjorn Helgaas wrote:
> 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?
>
I think this is the safest route.  PF<->VF interaction isn't architected,
and resetting the PF with active VFs will probably hang a number of SRIOV
implementations, requiring a system-level reset to correct the compounded problem.

>>>> 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.
>
+1.

> Bjorn
>

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