When commit 361c8dc17 added support for hotplugging the i6300esb watchdog device (first in libvirt-3.9.0), it accidentally contstructed the commandline for the device_add command before allocating a PCI address for the device. With no PCI address specified in the command, the watchdog would simply be placed at the lowest unused PCI slot. On a 440fx guest, this doesn't cause a problem, because libvirt's PCI address allocation algorithm would most likely give the same address anyway (usually a slot on pci-root), so nobody noticed the omission of address from the command. But on a Q35 guest, the lowest unused PCI slot is on pcie-root, which doesn't support hotplug; libvirt knows enough to assign a PCI address that is on a pcie-to-pci-bridge (because its slots *do* support hotplug), but qemu doesn't, so if there is no PCI address in the command, qemu just tries to plug the new device into pcie-root, and fails because it doesn't support hotplug, e.g.: error: Failed to attach device from watchdog.xml error: internal error: unable to execute QEMU command 'device_add': Bus 'pcie.0' does not support hotplugging The solution is simply to build the command string after assigning a PCI address, not before. Resolves: https://bugzilla.redhat.com/1666559 Signed-off-by: Laine Stump <laine(a)laine.org&gt; --- src/qemu/qemu_hotplug.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c index a1c3ca999b..40c693483e 100644 --- a/src/qemu/qemu_hotplug.c +++ b/src/qemu/qemu_hotplug.c @@ -3280,9 +3280,6 @@ qemuDomainAttachWatchdog(virQEMUDriverPtr driver, if (qemuAssignDeviceWatchdogAlias(watchdog) < 0) return -1; - if (!(watchdogstr = qemuBuildWatchdogDevStr(vm->def, watchdog, priv->qemuCaps))) - return -1; - if (watchdog->model == VIR_DOMAIN_WATCHDOG_MODEL_I6300ESB) { if (qemuDomainEnsurePCIAddress(vm, &dev, driver) < 0) goto cleanup; @@ -3294,6 +3291,9 @@ qemuDomainAttachWatchdog(virQEMUDriverPtr driver, goto cleanup; } + if (!(watchdogstr = qemuBuildWatchdogDevStr(vm->def, watchdog, priv->qemuCaps))) + goto cleanup; + /* QEMU doesn't have a 'dump' action; we tell qemu to 'pause', then libvirt listens for the watchdog event, and we perform the dump ourselves. so convert 'dump' to 'pause' for the qemu cli */ -- 2.20.1