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

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Date:	Tue, 11 Jun 2013 12:24:36 -0400
From:	konrad wilk <konrad.wilk@...cle.com>
To:	George Dunlap <george.dunlap@...citrix.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 6/11/2013 12:17 PM, 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.

Right.
>
> 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?
No. It was a feature.
>
>  -George

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