netdev - Re: [PATCH 6/7] PCI: Make sure VF's driver get attached after PF's

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Date:	Tue, 21 May 2013 18:09:06 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
CC:	Alexander Duyck <alexander.h.duyck@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Gu Zheng <guz.fnst@...fujitsu.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	NetDev <netdev@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 6/7] PCI: Make sure VF's driver get attached after PF's

On 05/21/2013 05:58 PM, Alexander Duyck wrote:
> On 05/21/2013 02:31 PM, Don Dutile wrote:
>> On 05/21/2013 05:30 PM, Don Dutile wrote:
>>> On 05/14/2013 05:39 PM, Alexander Duyck wrote:
>>>> On 05/14/2013 12:59 PM, Yinghai Lu wrote:
>>>>> On Tue, May 14, 2013 at 12:45 PM, Alexander Duyck
>>>>> <alexander.h.duyck@...el.com>  wrote:
>>>>>> On 05/14/2013 11:44 AM, Yinghai Lu wrote:
>>>>>>> On Tue, May 14, 2013 at 9:00 AM, Alexander Duyck
>>>>>>> <alexander.h.duyck@...el.com>  wrote:
>>>>>>>> I'm sorry, but what is the point of this patch? With device
>>>>>>>> assignment
>>>>>>>> it is always possible to have VFs loaded and the PF driver unloaded
>>>>>>>> since you cannot remove the VFs if they are assigned to a VM.
>>>>>>> unload PF driver will not call pci_disable_sriov?
>>>>>> You cannot call pci_disable_sriov because you will panic all of the
>>>>>> guests that have devices assigned.
>>>>> ixgbe_remove did call pci_disable_sriov...
>>>>>
>>>>> for guest panic, that is another problem.
>>>>> just like you pci passthrough with real pci device and hotremove the
>>>>> card in host.
>>>>>
>>>>> ...
>>>>
>>>> I suggest you take another look. In ixgbe_disable_sriov, which is the
>>>> function that is called we do a check for assigned VFs. If they are
>>>> assigned then we do not call pci_disable_sriov.
>>>>
>>>>>
>>>>>> So how does your patch actually fix this problem? It seems like it is
>>>>>> just avoiding it.
>>>>> yes, until the first one is done.
>>>>
>>>> Avoiding the issue doesn't fix the underlying problem and instead you
>>>> are likely just introducing more bugs as a result.
>>>>
>>>>>>  From what I can tell your problem is originating in pci_call_probe. I
>>>>>> believe it is calling work_on_cpu and that doesn't seem correct since
>>>>>> the work should be taking place on a CPU already local to the PF. You
>>>>>> might want to look there to see why you are trying to schedule work
>>>>>> on a
>>>>>> CPU which should be perfectly fine for you to already be doing your
>>>>>> work on.
>>>>> it always try to go with local cpu with same pxm.
>>>>
>>>> The problem is we really shouldn't be calling work_for_cpu in this case
>>>> since we are already on the correct CPU. What probably should be
>>>> happening is that pci_call_probe should be doing a check to see if the
>>>> current CPU is already contained within the device node map and if so
>>>> just call local_pci_probe directly. That way you can avoid deadlocking
>>>> the system by trying to flush the CPU queue of the CPU you are
>>>> currently on.
>>>>
>>> That's the patch that Michael Tsirkin posted for a fix,
>>> but it was noted that if you have the case where the _same_ driver is
>>> used for vf&  pf,
>>> other deadlocks may occur.
>>> It would work in the case of ixgbe/ixgbevf, but not for something like
>>> the Mellanox pf/vf driver (which is the same).
>>>
>> apologies; here's the thread the discussed the issue:
>> https://patchwork.kernel.org/patch/2458681/
>>
>
> I found out about that patch after I submitted one that was similar.
> The only real complaint I had about his patch was that it was only
> looking at the CPU and he could save himself some trouble by just doing
> the work locally if we were on the correct NUMA node.  For example if
> the system only has one node in it what is the point in scheduling all
> of the work on CPU 0?  My alternative patch can be found at:
> https://patchwork.kernel.org/patch/2568881/
>
> As far as the inter-driver locking issues for same driver I don't think
> that is really any kind if issue.  Most drivers shouldn't be holding any
> big locks when they call pci_enable_sriov.  If I am not mistaken the
> follow on patch I submitted which was similar to Michaels was reported
> to have resolved the issue.
>
You mean the above patchwork patch, or another one?

> As far as the Mellanox PF/VF the bigger issue is that when they call
> pci_enable_sriov they are not ready to handle VFs.  There have been
> several suggestions on how to resolve it including -EPROBE_DEFER or the
> igbvf/ixgbevf approach of just brining up the device in a "link down" state.
>
thanks for summary.  i was backlogged on email, and responding as i read them;
I should have read through the whole thread before chiming in.

> Thanks,
>
> Alex
>
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