On Fri, Jul 11, 2025 at 01:49:28PM +0100, Mark Cave-Ayland wrote: > On 09/07/2025 13:57, Daniel P. Berrangé wrote: > > > On Wed, Jul 09, 2025 at 01:42:20PM +0100, Mark Cave-Ayland wrote: > > > On 09/07/2025 12:10, Daniel P. Berrangé wrote: > > > > > > > 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. > > > > > > Thanks for the update, I'll take a look at implementing something along > > > these lines. Would it be slightly easier to add the guest-visible UUID as an > > > attribute to the existing uuid element e.g. > > > > > > <uuid guest-uuid="....">....</uuid> > > > > > > since then it should already be accessible at the places where we already > > > have an existing reference to the uuid element? > > > > We already have the 'genid' UUID as a separate element, so I figure > > we just carry on that practice with 'hwuuid'. > > Thanks for the hints! I've been looking at the various drivers within > libvirt, and from what I can see not all virtualisation platforms will allow > a separate UUID to be provided in this way. > > Would the best way forward be to add a new capability to represent this > ability, and then follow up with a QEMU implementation? I think its probably overkill to add capability. Just define the XML and wire up QEMU. With 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 :|
