In a case where we want to hotplug the following disk: <disk type='file' device='disk'> (...) <address type='drive' controller='0' bus='0' target='0' unit='0'/> </disk> In a QEMU guest that has a single OS disk, as follows: <disk type='file' device='disk'> (...) <address type='drive' controller='0' bus='0' target='0' unit='0'/> </disk> What happens is that the existing guest disk will receive the ID 'scsi0-0-0-0' due to how Libvirt calculate the alias based on the address in qemu_alias.c, qemuAssignDeviceDiskAlias. When hotplugging a disk that happens to have the same address, Libvirt will calculate the same ID to it and attempt to device_add. QEMU will refuse it: $ virsh attach-device dhb hp-disk-dup.xml error: Failed to attach device from hp-disk-dup.xml error: internal error: unable to execute QEMU command 'device_add': Duplicate ID 'scsi0-0-0-0' for device And Libvirt follows it up with a cleanup code in qemuDomainAttachDiskGeneric that ends up removing what supposedly is a faulty hotplugged disk but, in this case, ends up being the original guest disk. This happens because Libvirt doesn't differentiate the error received by QMP device_add. An argument can be made for how QMP device_add should provide a different error code for this scenario. A quicker way to solve the problem is presented in this patch: let us check the generated alias against the aliases already presented in the disks in the VM definition. If a match happens, error out without calling device_add. After this patch, this is the result of the previous attach-device call: $ ./run tools/virsh attach-device dhb ~/hp-disk-dup.xml [sudo] password for danielhb: error: Failed to attach device from /home/danielhb/hp-disk-dup.xml error: operation failed: attached disk conflicts with existing device id 'scsi0-0-0-0' Reported-by: Srikanth Aithal <bssrikanth(a)in.ibm.com&gt; Signed-off-by: Daniel Henrique Barboza <danielhb413(a)gmail.com&gt; --- src/qemu/qemu_hotplug.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c index a1c3ca999b..7c770211ab 100644 --- a/src/qemu/qemu_hotplug.c +++ b/src/qemu/qemu_hotplug.c @@ -875,6 +875,27 @@ qemuDomainChangeEjectableMedia(virQEMUDriverPtr driver, } +/** + * qemuDomainDeviceAliasIsUnique: + * + * Searches the existing domain disks to check if a given alias is + * unique. */ +static bool +qemuDomainDeviceAliasIsUnique(virDomainDefPtr def, char *alias) +{ + int idx; + + for (idx = (def->ndisks - 1); idx >= 0; idx--) { + if (def->disks[idx]->info.alias && + (strcmp(def->disks[idx]->info.alias, alias) == 0)) { + + return false; + } + } + return true; +} + + /** * qemuDomainAttachDiskGeneric: * @@ -897,6 +918,13 @@ qemuDomainAttachDiskGeneric(virQEMUDriverPtr driver, if (qemuAssignDeviceDiskAlias(vm->def, disk, priv->qemuCaps) < 0) goto error; + if (!qemuDomainDeviceAliasIsUnique(vm->def, disk->info.alias)) { + virReportError(VIR_ERR_OPERATION_FAILED, + _("attached disk conflicts with existing " + "device id '%s'"), disk->info.alias); + goto error; + } + if (qemuDomainPrepareDiskSource(disk, priv, cfg) < 0) goto error; -- 2.20.1