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2f2f709a0e
Extending the snapshot/restore documentation so that it explains what are the goals behind this offloaded snapshot/restore feature, how to use it in practice, and also by documenting the protocol used by the offload daemon so that anyone could write its own daemon. By relying on the existing local live migration support and reusing the semantics and the protocol associated with it, we intend to provide a way for snapshotting and restoring a VM to/from a dedicated process that we can call the offload daemon. By allowing an external process to perform the snapshot/restore actions on behalf of Cloud Hypervisor, we give our users the opportunity to implement their own offloaded daemon. The goal is to avoid bloating Cloud Hypervisor with numerous features related to snapshot/restore, and let the user decide how to perform the snapshot/restore actions. One example is that we can decide to encrypt the guest RAM on the fly in order to avoid writing an unencrypted version to local disk. Another example is to be able to send guest RAM and associated state/config data over the network without having to persist the data first to local storage. There might be other reasons to choose going with an offloaded daemon to perform the snapshot/restore of the VM, but in every case, this empowers the user to make their own choice. Signed-off-by: Sebastien Boeuf <sboeuf@meta.com> Assisted-by: Claude:claude-opus-4-7
258 lines
9.4 KiB
Markdown
258 lines
9.4 KiB
Markdown
# Snapshot and Restore
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The goal for the snapshot/restore feature is to provide the user with the
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ability to take a snapshot of a previously paused virtual machine. This
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snapshot can be used as the base for creating new identical virtual machines,
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without the need to boot them from scratch. The restore codepath takes the
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snapshot and creates the exact same virtual machine, restoring the previously
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saved states. The new virtual machine is restored in a paused state, as it was
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before the snapshot was performed.
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## Snapshot a Cloud Hypervisor VM
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First thing, we must run a Cloud Hypervisor VM:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--cpus boot=4 \
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--memory size=4G \
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--kernel vmlinux \
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--cmdline "root=/dev/vda1 console=hvc0 rw" \
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--disk path=focal-server-cloudimg-amd64.raw
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```
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At any point in time when the VM is running, one might choose to pause it:
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock pause
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```
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Once paused, the VM can be safely snapshot into the specified directory and
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using the following command:
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock snapshot file:///home/foo/snapshot
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```
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Given the directory was present on the system, the snapshot will succeed and
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it should contain the following files:
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```bash
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ll /home/foo/snapshot/
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total 4194536
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drwxrwxr-x 2 foo bar 4096 Jul 22 11:50 ./
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drwxr-xr-x 47 foo bar 4096 Jul 22 11:47 ../
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-rw------- 1 foo bar 1084 Jul 22 11:19 config.json
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-rw------- 1 foo bar 4294967296 Jul 22 11:19 memory-ranges
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-rw------- 1 foo bar 217853 Jul 22 11:19 state.json
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```
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`config.json` contains the virtual machine configuration. It is used to create
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a similar virtual machine with the correct amount of CPUs, RAM, and other
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expected devices. It is stored in a human readable format so that it could be
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modified between the snapshot and restore phases to achieve some very special
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use cases. But for most cases, manually modifying the configuration should not
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be needed.
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`memory-ranges` stores the content of the guest RAM.
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`state.json` contains the virtual machine state. It is used to restore each
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component in the state it was left before the snapshot occurred.
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## Restore a Cloud Hypervisor VM
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Given that one has access to an existing snapshot in `/home/foo/snapshot`,
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it is possible to create a new VM based on this snapshot with the following
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command:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--restore source_url=file:///home/foo/snapshot
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```
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Or using two different commands from two terminals:
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```bash
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# First terminal
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./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock
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# Second terminal
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot
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```
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Remember the VM is restored in a `paused` state, which was the VM's state when
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it was snapshot. For this reason, one must explicitly `resume` the VM before to
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start using it.
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock resume
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```
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Alternatively, the `resume` option can be used to automatically resume the VM
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after restore completes:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--restore source_url=file:///home/foo/snapshot,resume=true
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```
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At this point, the VM is fully restored and is identical to the VM which was
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snapshot earlier.
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Restore also supports selecting how guest memory is populated:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--restore source_url=file:///home/foo/snapshot,memory_restore_mode=ondemand
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```
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If `memory_restore_mode` is omitted, Cloud Hypervisor uses the eager-copy
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restore path (`copy`).
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With `memory_restore_mode=ondemand`, restore uses `userfaultfd` to fault snapshot
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pages in on first access instead of copying the full `memory-ranges` file into
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guest RAM before restore completes. This mode is strict: if Cloud Hypervisor
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cannot enable the `userfaultfd` restore path, restore fails instead of falling
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back to `copy`.
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Current constraints for `memory_restore_mode=ondemand`:
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- `prefault=on` is not supported
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- the snapshot memory ranges must be page-aligned
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## Restore a VM with new Net FDs
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For a VM created with FDs explicitly passed to NetConfig, a set of valid FDs
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need to be provided along with the VM restore command in the following syntax:
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```bash
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# First terminal
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./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock
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# Second terminal
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot net_fds=[net1@[23,24],net2@[25,26]]
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```
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In the example above, the net device with id `net1` will be backed by FDs '23'
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and '24', and the net device with id `net2` will be backed by FDs '25' and '26'
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from the restored VM.
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## Limitations
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VFIO devices is out of scope.
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## Offload Snapshot and Restore
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Cloud Hypervisor can hand the snapshot payload off to a user-provided
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offload daemon instead of writing files to a `file://` directory. The
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daemon can transform the payload on the fly (encrypt, compress, stream
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to object storage, etc.) without ever touching local disk.
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There is no dedicated API surface for offload: the daemon talks to CH
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over the existing local live-migration protocol, playing the migration
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peer role:
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- On snapshot, CH acts as the migration sender and the daemon acts as the
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receiver. The source VM shuts down on success, exactly as it would for a
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local live migration. Memory is transferred via `SCM_RIGHTS`, CH handing
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off the daemon one memfd per guest-memory slot.
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- On restore, CH acts as the migration receiver and the daemon acts as the
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sender. The daemon provides one memfd per slot, populated from its
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storage, and CH uses those memfds directly as guest RAM backing.
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In practice, this means offload is driven through the existing
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`vm.send-migration` / `vm.receive-migration` endpoints (with `local=on`
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and a `unix:<path>` URL). The daemon is just another peer of these
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endpoints. This requires the VM to be configured with shared-memory
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backing, which is the same precondition that applies to local live
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migration today.
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### Snapshot offload usage
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```bash
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# 1. Run a VM with shared memory.
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--cpus boot=2 \
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--memory size=1G,shared=on \
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--kernel vmlinux \
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--cmdline "root=/dev/vda1 console=hvc0 rw" \
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--disk path=focal-server-cloudimg-amd64.raw
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# 2. Start your offload daemon. The reference implementation is shipped as
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# `offload_daemon` and persists snapshot data to a local directory.
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./offload_daemon snapshot \
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--socket /tmp/offload.sock \
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--output-dir /var/snapshots/vm1
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# 3. Issue a local live migration to the daemon's socket. CH connects to
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# /tmp/offload.sock, streams the snapshot, and exits on success.
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./ch-remote --api-socket /tmp/cloud-hypervisor.sock pause
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./ch-remote --api-socket /tmp/cloud-hypervisor.sock \
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send-migration destination_url=unix:/tmp/offload.sock,local=on
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```
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### Restore offload usage
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```bash
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# 1. Start a CH process.
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./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock
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# 2. Tell CH to listen for an inbound migration from the offload daemon.
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./ch-remote --api-socket /tmp/cloud-hypervisor.sock \
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receive-migration receiver_url=unix:/tmp/restore.sock &
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# 3. Start the daemon in restore mode pointing at the same saved snapshot.
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# With --resume, the restored VM starts running on completion;
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# without it, the VM is left paused (issue `resume` to start it).
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./offload_daemon restore \
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--socket /tmp/restore.sock \
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--input-dir /var/snapshots/vm1 \
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--resume
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```
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### The daemon protocol
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The daemon implements the local live-migration wire protocol defined in
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`vm-migration/src/protocol.rs`. Two state machines are involved:
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- Snapshot mode (migration receiver): walk
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`Start → MemoryFd (×N) → Config → State → CompletePaused`. For each
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`MemoryFd` command, receive a guest-memory fd via SCM_RIGHTS on the
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same UNIX socket.
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- Restore mode (migration sender): walk the same sequence in reverse,
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emitting one `MemoryFd` per slot (with the memfd attached via SCM_RIGHTS)
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before sending `Config` and `State`. Finish with either `CompletePaused`
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(restored VM remains paused) or `Complete` (restored VM resumes).
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### Critical invariant on snapshot
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On the snapshot path, the daemon must finish reading from every memory fd
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before it ACKs `CompletePaused`. Cloud Hypervisor blocks at the
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`CompletePaused` handshake until the daemon ACKs. Once it ACKs, the source
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VM shuts down and the daemon's fds are the only remaining record of guest
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RAM. The reference daemon dumps each slot to disk and `fsync`s before
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ACKing.
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### Reference daemon
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The in-tree `offload_daemon` binary is intentionally minimal: it just
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serialises the snapshot to a local directory and replays it back. Its
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purpose is to back the offload integration test and to serve as a
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working example for daemon authors. Use it as a template, not a
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production backend.
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### Limitations
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- The VM must use shared-memory backing (`shared=on` or file-backed).
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Anonymous memory is rejected with the same error message that local
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live migration produces.
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- Orchestrator-supplied network FDs (today carried by `vm.restore`'s
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`net_fds` field) are not plumbed through `vm.receive-migration`,
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so VMs whose configuration relies on externally-provided net FDs
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cannot currently be restored via the offload path.
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- Confidential VMs (CVMs) inherit the live-migration restriction: offload
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is not supported for CVMs.
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