On Wed, Jul 09, 2025 at 11:27:45AM +0100, Daniel P. Berrangé via Devel wrote:
On Mon, Jul 07, 2025 at 02:19:10PM +0100, Mark Cave-Ayland wrote:
Hi all,
I'm currently looking at how libvirt can be used to clone a saved VM, and
have been focusing on the previous thread on this topic at
https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.libvirt.org_archives_list_devel-40lists.libvirt.org_thread_YXN2L2PYL4V4576ON5OKQKCV7RCWPSCT_-23PJWWXVWIXB3L6DYUZDFW5DWIBFBBW3WS&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=c23RpsaH4D2MKyD3EPJTDa0BAxz6tV8aUJqVSoytEiY&m=7ZXi_PM9XP0VGHkNp6YBHF9EgJtqBO0Yes05I1hCcQ6WXAGvRu1GrhubXCAk9GlZ&s=A-lBN3Je6zF4x-xBmIrfGzVBbEzvUWQfNU-BZMkngy0&e=
.
My understanding from reading the thread is that the best way to approach
this is 1) add uuid properties to the relevant objects/devices in QEMU where
the UUID is user-visible, and 2) update libvirt to handle these properties
via XML.
Does this sound correct, or is there any more recent thinking about the best
approach to take?
Yes, that's still a reasonable way to approach this, unless we uncover
new info when you try implementing it.
Well it didn't take me long to uncover new info :-(
Looking at the QEMU codebase for where 'qemu_uuid' is used reveals rather
more than anticipated
git grep -E '\bqemu_uuid(_set)?\b'
hw/core/machine-qmp-cmds.c: info->UUID =
qemu_uuid_unparse_strdup(&qemu_uuid);
hw/i386/vmport.c: uint32_t *uuid_parts = (uint32_t *)(qemu_uuid.data);
hw/nvram/fw_cfg.c: fw_cfg_add_bytes(s, FW_CFG_UUID, &qemu_uuid, 16);
hw/ppc/pnv.c: buf = qemu_uuid_unparse_strdup(&qemu_uuid);
hw/ppc/pnv.c: if (qemu_uuid_set) {
hw/ppc/spapr.c: buf = qemu_uuid_unparse_strdup(&qemu_uuid);
hw/ppc/spapr.c: if (qemu_uuid_set) {
hw/ppc/spapr_rtas.c: ret = sysparm_st(buffer, length, (unsigned char
*)&qemu_uuid,
hw/ppc/spapr_rtas.c: (qemu_uuid_set ? 16 : 0));
hw/smbios/smbios.c: if (qemu_uuid_set) {
hw/smbios/smbios.c: smbios_encode_uuid(&t->uuid, &qemu_uuid);
hw/smbios/smbios.c: if (qemu_uuid_parse(val, &qemu_uuid) != 0) {
hw/smbios/smbios.c: qemu_uuid_set = true;
hw/smbios/smbios_legacy.c: if (qemu_uuid_set) {
hw/smbios/smbios_legacy.c: &qemu_uuid, 16);
include/hw/firmware/smbios.h: /* uuid is in qemu_uuid */
include/sysemu/sysemu.h:extern QemuUUID qemu_uuid;
include/sysemu/sysemu.h:extern bool qemu_uuid_set;
migration/multifd.c: memcpy(msg.uuid, &qemu_uuid.data, sizeof(msg.uuid));
migration/multifd.c: if (memcmp(msg.uuid, &qemu_uuid, sizeof(qemu_uuid))) {
migration/multifd.c: char *uuid = qemu_uuid_unparse_strdup(&qemu_uuid);
migration/savevm.c: state->uuid = qemu_uuid;
migration/savevm.c: return qemu_uuid_set && migrate_validate_uuid();
migration/savevm.c: if (!qemu_uuid_set) {
migration/savevm.c: if (!qemu_uuid_is_equal(&state->uuid, &qemu_uuid)) {
migration/savevm.c: qemu_uuid_unparse(&qemu_uuid, uuid_dst);
system/globals.c:/* The bytes in qemu_uuid are in the order specified by
RFC4122, _not_ in the
system/globals.c:QemuUUID qemu_uuid;
system/globals.c:bool qemu_uuid_set;
system/vl.c: if (qemu_uuid_parse(optarg, &qemu_uuid) < 0) {
system/vl.c: qemu_uuid_set = true;
target/s390x/kvm/kvm.c: memcpy(sysib.vm[0].uuid, &qemu_uuid,
sizeof(sysib.vm[0].uuid));
target/s390x/tcg/misc_helper.c: memcpy(sysib.sysib_322.vm[0].uuid,
&qemu_uuid,
ui/dbus.c: g_autofree char *uuid = qemu_uuid_unparse_strdup(&qemu_uuid);
ui/spice-core.c: spice_server_set_uuid(spice_server, (unsigned char
*)&qemu_uuid);
Only a small subset of those places have a directly exposed property that
could override the default they obtain from the -uuid arg.
The savevm code in particular is a sticking point as we need to use
the migration code as the mechanism for cloning.
So if we want to enable cloning, without the guest visible UUIDs changing,
and with the savevm checks passing, we're pretty much stuck with having to
set -uuid to a *different* value than the <uuid></uuid> in the XML :-(
This probably takes us back to needing to have another element at the
top level in the XML.
<uuid>....</uuid>
<hwuuid>...</hwuuid>
where <uuid> is mandatory as today, and <hwuuid> can optionally be used
to override what is exposed to the guest, defaulting to <uuid> if
omitted.
