linux-kernel - Re: [PATCH] xen/pci: Deal with toolstack missing an 'XenbusStateClosing'.

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Message-ID: <51B87BDF.1080101@eu.citrix.com>
Date:	Wed, 12 Jun 2013 14:47:11 +0100
From:	George Dunlap <george.dunlap@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	<linux-pci@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH] xen/pci: Deal with toolstack missing an 'XenbusStateClosing'.

On 12/06/13 14:45, Konrad Rzeszutek Wilk wrote:
> On Tue, Jun 11, 2013 at 05:17:45PM +0100, George Dunlap wrote:
>> On 06/11/2013 05:08 PM, konrad wilk wrote:
>>> On 6/11/2013 11:36 AM, George Dunlap wrote:
>>>> On 06/10/2013 10:06 PM, Konrad Rzeszutek Wilk wrote:
>>>>> There are two tool-stack that can instruct the Xen PCI frontend
>>>>> and backend to change states: 'xm' (Python code with a daemon),
>>>>> and 'xl' (C library - does not keep state changes).
>>>>>
>>>>> With the 'xm', the path to disconnect a PCI device (xm pci-detach
>>>>> <guest> <BDF>)is:
>>>>>
>>>>> 4(Connected)->7(Reconfiguring*)-> 8(Reconfigured)->
>>>>> 4(Connected)->5(Closing*).
>>>>>
>>>>> The * is for states that the tool-stack sets. For 'xl', it is similar:
>>>>>
>>>>> 4(Connected)->7(Reconfiguring*)-> 8(Reconfigured)-> 4(Connected)
>>>>>
>>>>> Both of them also tear down the XenBus structure, so the backend
>>>>> state ends up going in the 3(Initialised) and calls
>>>>> pcifront_xenbus_remove.
>>>> So I looked a little bit into this; there are actually two different
>>>> states that happen as part of this handshake.  In order to disonnect a
>>>> *device*, xl signals using the *bus* state, like this:
>>>> * Wait for the *bus* to be in state 4(Connected)
>>>> * Set the *device* state to 5(Closing)
>>>> * Set the *bus* state to 7(Reconfiguring)
>>>> * Wait for the *bus* state to return to 4(Connected)
>>>>
>>>> So are all of these states you see the *bus* state?  And why would you
>>>> disconnect the whole pci bus if you're only removing one device?
>>> Correct. The stats I enumerated are *bus* states. Not per-device states.
>>> I presume (and I hadn't checked xm) that Xend has some logic to only
>>> disconnect the bus if all of the PCI devices have been disconnected. In
>>> 'xl' it does not do that.
>>>
>>> The testing I did was just with one PCI device.
>> Ah, OK -- I see now.  The problem is that the code in the Linux side
>> didn't know about the whole "4->7->8->4" thing to unplug a device.
>> In all likelihood, if you had used xm with two devices (so that the
>> bus didn't get disconnected), then you would have run across the
>> same error.
>>
>> So at least part of the problem *is* a bug in Linux.
> Good! Bjorn, would you be OK Ack-ing the patch I sent (attached here
> for reference) or putting it in your queue for Linus?
>
> My plan would be to send it to Linus in the 3.11 merge window.

One nit -- "to work with the 'xl' toolstack" -- didn't we theorize this 
would also be broken with xm if you had two devices passed through?

  -George
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