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

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Date:	Tue, 11 Jun 2013 17:17:45 +0100
From:	George Dunlap <george.dunlap@...citrix.com>
To:	konrad 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 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.

That doesn't explain why I have problems doing this on Debian's version 
of 3.2 -- unless the "fix" you mentoned above was backported to the 
stable kernel, perhaps?

  -George
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