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Subject: virtio-vsock live migration

Michael pointed out that the virtio-vsock draft specification does not
address live migration and in fact currently precludes migration.

Migration is fundamental so the device specification at least mustn't
preclude it.  Having brainstormed migration with Matthew Benjamin and
Michael Tsirkin, I am now summarizing the approach that I want to
include in the next draft specification.

Feedback and comments welcome!  In the meantime I will implement this in
code and update the draft specification.

1. Requirements

Virtio-vsock is a new AF_VSOCK transport.  As such, it should provide at
least the same guarantees as the existing AF_VSOCK VMCI transport.  This
is for consistency and to allow code reuse across any AF_VSOCK

Virtio-vsock aims to replace virtio-serial by providing the same
guest/host communication ability but with sockets API semantics that are
more popular and convenient for application developers.  Therefore
virtio-vsock migration should provide at least the same level of
migration functionality as virtio-serial.

Ideally it should be possible to migrate applications using AF_VSOCK
together with the virtual machine so that guest<->host communication is
interrupted.  Neither AF_VSOCK VMCI nor virtio-serial support this

2. Basic disruptive migration flow

When the virtual machine migrates from the source host to the
destination host, the guest's CID may change.  The CID namespace is
host-wide so other hosts may have CID collisions and allocate a new CID
for incoming migration VMs.

The device notifies the guest that the CID has changed.  Guest sockets
are affected as follows:

 * Established connections are reset (ECONNRESET) and the guest
   application will have to reconnect.

 * Listen sockets remain open.  The only thing to note is that
   connections from the host are now made to the new CID.  This means
   the local address of the listen socket is automatically updated to
   the new CID.

 * Sockets in other states are unchanged.

Applications must handle disruptive migration by reconnecting if
necessary after ECONNRESET.

3. Checkpoint/restore for seamless migration

Applications that wish to communicate across live migration can do so
but this requires extra application-specific checkpoint/restore code.

This is similar to the approach taken by the CRIU project where
getsockopt()/setsockopt() is used to migrate socket state.  The
difference is that the application process is not automatically migrated
from the source host to the destination host.  Therefore, the
application needs to migrate its own state somehow.

The flow is as follows:

The application on the source host must quiesce (stop sending/receiving)
and use getsockopt() to extract socket state information from the host

A new instance of the application is started on the destination host and
given the state so it can restore the connection.  The setsockopt()
syscall is used to restore socket state information.

The guest is given a list of <host_old_cid, host_new_cid, host_port,
guest_port> tuples for established connections that must not be reset
when the guest CID update notification is received.  These connections
will carry on as if nothing changed.

Note that the connection's remote address is updated from host_old_cid
to host_new_cid.  This allows remapping of CIDs (if necessary).
Typically this will be unused because the host always has well-known CID
2.  In a guest<->guest scenario it may be used to remap CIDs.

For the time being I am focussing on the basic disruptive migration flow
only.  Checkpoint/restore can be added with a feature bit in the future.
It is a lot more complex and I'm not sure whether there will be any
users yet.


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