Re: [libvirt] RFC: revival of hotplug/unplug for PCI Multifunction devices in QEMU guests

Hi, This is labeled as RFC but it's more like a FYI to let people know and comment beforehand. Shiva sent a 28 patch series last year that implements hotplug/unplug support for PCI multifunction devices [1]. The design motivation of his work was based in a RFC sent to this mailing list back in 2016 [2]. I'll briefly summarize the goals and motivations here. What we have today in Libvirt: - no hotplug/unplug support for multifunction PCI devices This is explained in details in [2]. When hotplugging a multifunction device, QEMU will queue the hotplug operation of all non-zero functions and, when function 0 is hotplugged, all functions are hotplugged together. This is true for all archs that supports PCI multifunction devices in QEMU. For unplug it varies: x86 will unplug all functions if any function is unplugged, ppc64 needs to unplug each one. Due to the nature of how Libvirt hotplug works now, hotplug of these devices is not possible. All hotplugs are considered in an isolated manner. Even if we hotplug each function in the proper order (i.e. leaving function 0 last), Libvirt can assign different slots in the guest for each. Similar problems happens with hot-unplug. This feature aims to address these by creating a new <devices> element, exclusive for multifunction devices, that aggregates all functions of a device in a single operation. To handle this new element, the existing attach/detach functions in the QEMU driver now handles multiple devices. Attaching/detaching a single device is routed away from the specialized multifunction code to be handled to the existing attach/detach code base. - no support for partial assignment of functions We can't make the assumption that the guest will always assign all devices of a multifunction device. Some functions might be a security risk to expose to a guest, or the device can behave differently depending on the amount of functions assigned. Even if the 'leftover' functions can't be used to anything else in the host, the decision of full/partial assignment of functions should come from the user, not us. We can't predict how any other hardware vendor will setup its devices. This patch series also handles this case. ------ The latest version of this feature, rebase to cdd362e0e7a (the current master as I'm writing this), can be found at: https://github.com/danielhb/libvirt/tree/multifunction-rc2 This is a work still ongoing that it's not ready for contribution yet (first patches that changes the unit test code are breaking existing tests).