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

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Message-ID: <51B6E711.2000001@eu.citrix.com>
Date:	Tue, 11 Jun 2013 10:00:01 +0100
From:	George Dunlap <george.dunlap@...citrix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	xen-devel <xen-devel@...ts.xen.org>,
	<linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	<stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/pci: Deal with toolstack missing an 'XenbusStateClosing'.

On 06/11/2013 08:29 AM, Jan Beulich wrote:
>>>> On 10.06.13 at 23:06, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> 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.

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

  -George

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