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[qemu-proc-spec] Add -uuid to the arguments #4733

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[qemu-proc-spec] Add -uuid to the arguments #4733

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ad38309
[qemu-proc-spec] Add `-uuid` to the arguments
xmkg Mar 16, 2026
a9d4443
[qemu-proc-spec] Update unit test
xmkg Mar 16, 2026
5740cf5
[qemu-proc-spec] Preserve stream format
xmkg Mar 16, 2026
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8 changes: 6 additions & 2 deletions src/platform/backends/qemu/qemu_vm_process_spec.cpp
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@ricab ricab Mar 16, 2026

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This covers only the QEMU backend. What happens in other backends? Is a name-based UUID available in the new Hyper-V and AppleVZ today? If not, that's probably worth some wider team awareness and synchronizing.

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@xmkg xmkg Mar 16, 2026

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It should be available for all off-the-shelf hypervisors already -- the existing hyper-v implementation already does that:

ubuntu@test-vm-3:~$ sudo cat /sys/class/dmi/id/product_uuid
e190e5e1-6261-40ca-a806-1a197a1d4ca0

I don't know the specifics for Apple VZ, but I think there should be a way to set it, as it's a pretty common thing for hypervisors to do. https://developer.apple.com/documentation/virtualization/vzgenericmachineidentifier looks promising, maybe @sharder996 could confirm that. I'll check the new Hyper-V implementation to make sure that it's still available.

Original file line number Diff line number Diff line change
Expand Up @@ -38,7 +38,6 @@ mp::QemuVMProcessSpec::QemuVMProcessSpec(const mp::VirtualMachineDescription& de
QStringList mp::QemuVMProcessSpec::arguments() const
{
QStringList args;

if (resume_data)
{
args = resume_data->arguments;
Expand All @@ -63,10 +62,12 @@ QStringList mp::QemuVMProcessSpec::arguments() const
}
else
{
// The UUID needs to be unique per VM and must be consistent across boots.
const auto vm_uuid = utils::make_uuid(desc.vm_name);
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@ricab ricab Mar 16, 2026

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A few questions regarding effects on clones:

  • Since their name changes from the original, clones get a new UUID, correct?
  • Is that compatible with pre-existing disk contents in typical scenarios where this is relevant?
  • We cause cloud-init to rerun when cloning. Is that typically enough to cascade the UUID wherever relevant?

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@xmkg xmkg Mar 16, 2026

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Since their name changes from the original, clones get a new UUID, correct?

Yes. It should be treated as any other instance-unique field.

Is that compatible with pre-existing disk contents in typical scenarios where this is relevant?

Yes, it is abstracted from the disk contents. In reality, it's a hardware serial-like info burned into the BIOS by the manufacturer. No disk involvement at all.

We cause cloud-init to rerun when cloning. Is that typically enough to cascade the UUID wherever relevant?

The /sys/class/dmi/id/product_uuid file is immediately available to queries without cloud-init to be re-run.

auto mem_size =
QString::number(desc.mem_size.in_megabytes()) + 'M'; /* flooring here; format documented
in `man qemu-system`, under `-m` option; including suffix to avoid relying on default unit */

// clang-format off
args << platform_args;
// The VM image itself
args << "-device"
Expand Down Expand Up @@ -97,6 +98,9 @@ in `man qemu-system`, under `-m` option; including suffix to avoid relying on de
<< "-nographic";
// Cloud-init disk
args << "-cdrom" << desc.cloud_init_iso;
// To make `/sys/class/dmi/id/product_uuid` present
args << "-uuid" << vm_uuid;
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@ricab ricab Mar 16, 2026

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To implement instance renaming, we'll probably want to keep the UUID stable. We'd probably save it along with other instance metadata, right? Do you foresee any impediment doing that? Just to be sure.

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@xmkg xmkg Mar 16, 2026

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We'd probably save it along with other instance metadata, right?

That should be sufficient if we update the UUID on clone. I can't think of any potential gotchas on that.

// clang-format on
}

for (const auto& [_, mount_data] : mount_args)
Expand Down
4 changes: 3 additions & 1 deletion tests/unit/qemu/test_qemu_vm_process_spec.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -65,7 +65,7 @@ TEST_F(TestQemuVMProcessSpec, defaultArgumentsCorrect)
#else
const auto storage_interface = "virtio-scsi-pci";
#endif

const auto expected_uuid = multipass::utils::make_uuid(desc.vm_name);
EXPECT_EQ(spec.arguments(),
QStringList({"--enable-kvm",
"-nic",
Expand All @@ -89,6 +89,8 @@ TEST_F(TestQemuVMProcessSpec, defaultArgumentsCorrect)
"-nographic",
"-cdrom",
"/path/to/cloud_init.iso",
"-uuid",
expected_uuid,
"-virtfs",
"local,security_model=passthrough,uid_map=1000:1000,gid_map=1000:1000,"
"path=path/to/target,mount_tag=m810e457178f448d9afffc9d950d726"}));
Expand Down
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