On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote: > Although I've never experienced it, due to not running Windows guests, I've > recently learned that a Windows guest permits a user (hopefully only one > with local admin privileges??!) to "hot-unplug" any PCI device. I've also > learned that some hypervisor admins don't want to permit admins of the > virtual machines they're managing to unplug PCI devices. I believe this is > impossible to prevent on an i440fx-based machinetype, and can only be done > on a q35-based machinetype by assigning the devices to the root bus (so that > they are seen as integrated devices) rather than to a pcie-root-port. But > when libvirt is assigning PCI addresses to devices in a q35-base guest, it > will *always* assign a PCIe device to a pcie-root-port specifically so that > hotplug is possible (this was done to maintain functional parity with i440fx > guests, where all PCI slots support hotplug). After speaking with Alex & Laine on IRC, I learnt some further relevant points - In a "typical" physical machine PCI slots will not be marked as hotpluggable - /sys/bus/pci/slots only has 2 entries on my HP DL180, corresponding to unused physical PCI slots - QEMU is hardcoded to report all pci & pcie-root-ports as hotpluggable. So /sys/bus/pci/slots on i440fx has 31 entries, one for every device, while on q35 it has one entry for every pcie-root-port IIUC. - It is conceptually possible to enhance pcie-root-port device to allow its hotplug capability to be toggled. Alternatively a parallel pcie-root-port-nohotplug device could be created. The end result would be the same from guest POV - The vfio-pci device has a companion vfio-pci-nohotplug device. The difference is simply whether the QEMU DEviceClass has the "hotpluggable" attribute set, and is separate from whether the PCI(e) root port has hotplug enabled The last point here about vfio-pci si particularly important, as it shows libvirt needs to be capable of tracking hotpluggability independantly on the PCI port and the PCI device attached to the port. > 1) Rather than leaving the task of assignung the PCI addresses of devices to > libvirt (which is what essentially *all* management apps that use libvirt > currently do), the management application could itself directly assign the > PCI addressed of all devices to be slots on pcie.0. [snip] This is essentially a hack to work around the fact the the pcie-root-port is hardcoded to report itself as hotpluggable. As such I don't consider this is serious long term solution. If you absolutely cannot wait for a newer libvirt/QEMU, this solution could be used as a quick hack for mgmt apps, but long term we need todo better. > 2) libvirt could gain a knob "somewhere" in the domain XML to force a single > device, or all devices, to be assigned to a PCI address on pcie.0 rather > than on a pcie-root-port. This could be thought of as a "hint" about device > placement, as well as extra validation in the case that a PCI address has > been manually assigned. So, for example, let's say a "hotplug='disable'" > option is added somewhere at the top level of the domain (maybe "<hotplug > enable='no'/>" inside <features> or something like that); when PCI addresses > are assigned by libvirt, it would attempt to find a slot on a controller > that didn't support hotplug. And/or a similar knob could be added to each > device. In both cases, the setting would be used both when assigning PCI > addresses and also to validate user-provided PCI addresses to assure that > the desired criterion was met (otherwise someone would manually select a PCI > address on a controller that supported hotplug, but then set > "hotplug='disabled'" and expect hotplug to be magically disabled on the > slot). Essentially this is the using "hotpluggable=yes|no" on the device as a policy knob to control device placement, to again workaround the fact that pcie-root-port are hardcoded to always report themselves as hotpluggable. So I'd think this should be ruled out for the same reason as option 1. It has a second downside though. As we see from vfio-pci-nohotplug, there is a valid use case for a "hotpluggable=yes|no" attribute on a device for controlling a specific hardware config choice in QEMU. We don't want to overload this attribute to both control use of vfio-pci-nohotplug, and also be a policy knob for device placement. > 3) qemu could add a "hotpluggable=no" commandline option to all PCI devices > (including vfio-pci) and then do whatever is necessary to make sure this is > honored in the emulated hardware (is it possible to set this on a per-slot > basis in a PCI controller? Or must it be done for an entire controller? I > suppose it's not as much of an issue for pcie-root-port, as long as you're > not using multiple functions). libvirt would then need to add this option to > the XML for each device, and management applications would need to set it - > it would essentially look the same to the management application, but it > would be implemented differently - instead of libvirt using that flag to > make a choice about which slot to assign, it would assign PCI addresses in > the same manner as before, and use the libvirt XML flag to set a QEMU > commandline flag for the device. I think this, or something close to it is the desirable way forward here, as it gives us more explicit control over what the emulated hardware actually advertizes. So instead of trying to workaround limitations of QEMU, we'd be working /with/ QEMU to improve its feature offerings. In QEMU pcie-root-port either needs to gain a hotpluggable=yes|no attribute, or a second pcie-root-port-nohotplug needs adding. Withever of these two approaches are taken in QEMU, this can be controlled from libvirt via an attribute on the controller.eg <controller type='pci' model='pcie-root-port' hotpluggable="no|yes"/> This hotpluggable attribute can be mapped to whichever CLI syntax QEMU wants to support. This alone is probably sufficient for the Windows problem motivating this thread. There already exists the vfio-pci-nohotplug device, but this is not exposed by libvirt. So we can add an attribute to <hostdev> to control its use. The remaining question is whether there's any compelling reason to add non-hotpluggable variants of other devices, virtio-net-pci-nohotplug ? I'd probably /not/ do this, unless there's a clear compelling benefit it gives which can't be achieved already via the pcie-root-port hotpluggability controls. For management applications, with Q35 we already recommend that they explicitly add *many* <controller type='pci' model='pcie-root-port'/> to new guests. Enough to cover all the initial cold-plugged devices, and enough spare ports to enable future hotplug of extra devices. OpenStack for example will add 32 pcie-root-ports, so that Q35 has approximately the same hotplug capacity as i440fx would have offered. To control hotplug, management apps simply need tweak what they're doing with pcie-root-ports with an extra attribute eg, consider there were 4 devices on the initially booted VM which need hotplug disabled, and we still want freedom to hotplug 2 extra devices at runtime. <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/> This is quite easy, as applications still do *not* have to taken on responsibility for full PCI device addressing. They merely need to be able to count how many PCI devices they're using. The only "gotcha" is if they forget about the auto-added USB, VGA and Balloon devices, but that's not a big deal. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
