xen/pcifront: Deal with toolstack missing 'XenbusStateClosing' state.
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 single 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 back 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] Note that this is for a _single_ PCI device. If there were two PCI devices and only one was disconnected 'xm' would show the same state changes. The problem is that git commit 3d925320 ("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 (for one or more PCI devices) and with 'xm' toolstack (for more than two PCI devices). Acked-by: Bjorn Helgaas <firstname.lastname@example.org> Cc: email@example.com Cc: firstname.lastname@example.org [v2: Added in the description about two PCI devices] Signed-off-by: Konrad Rzeszutek Wilk <email@example.com>