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

On 6/11/2013 5:00 AM, George Dunlap wrote:
On 06/11/2013 08:29 AM, Jan Beulich wrote:
On 10.06.13 at 23:06, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 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.

When a PCI device is plugged in (xm pci-attach <guest> <BDF>)
both of them follow the same pattern:
2(InitWait*), 3(Initialized*), 4(Connected*)->4(Connected).

[xen-pcifront ignores the 2,3 state changes and only acts when
4 (Connected) has been reached]

The problem is that git commit 3d925320e9e2de162bd138bf97816bda8c3f71be
("xen/pcifront: Use Xen-SWIOTLB when initting if required") introduced
a mechanism to initialize the SWIOTLB when the Xen PCI front moves to
Connected state. It also had some aggressive seatbelt code check that
would warn the user if one tried to change to Connected state without
hitting first the Closing state:

  pcifront pci-0: PCI frontend already installed!

However, that code can be relaxed and we can continue on working
even if the frontend is instructed to be the 'Connected' state with
no devices and then gets tickled to be in 'Connected' state again.

In other words, this 4(Connected)->5(Closing)->4(Connected) state
was expected, while 4(Connected)->.... anything but 5(Closing)->4(Connected)
was not. This patch removes that aggressive check and allows
Xen pcifront to work with the 'xl' toolstack.

I actually think this shouldn't be worked around here, but fixed in
xl. Any device removed from a guest should be driven towards
the "Closed" state.

There is also the per-device state. Those are moved to the 5 (Closing), while the whole connection is still in the 4(Connected) state. In essence all of the per-device states
are closed, it is just that the global state is still Connected.

Yeah, that seems pretty obvious to me. The weird thing is that this wasn't noticed before -- does this work in 4.2? Have you been doing this test all along, or has it only broken recently?

I just reproduced this in Xen 4.2. I believe that the reason I did not see this before was b/c I was using 'xm'

I've reproduced it on one of my test boxes; let me see if I can sort it out.



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