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

From: George Dunlap
Date: Wed Jun 12 2013 - 09:47:31 EST

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)->

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
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?

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