Re: [PATCH V4 19/19] migration: cpr-transfer documentation

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 02, 2024 at 05:20:11AM -0800, Steve Sistare wrote:
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>

I think this one is already good enough, so:

Reviewed-by: Peter Xu <peterx@redhat.com>

But still, a few comments inline.

> ---
>  docs/devel/migration/CPR.rst | 176 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 174 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/devel/migration/CPR.rst b/docs/devel/migration/CPR.rst
> index 63c3647..a8a57c0 100644
> --- a/docs/devel/migration/CPR.rst
> +++ b/docs/devel/migration/CPR.rst
> @@ -5,7 +5,7 @@ CPR is the umbrella name for a set of migration modes in which the
>  VM is migrated to a new QEMU instance on the same host.  It is
>  intended for use when the goal is to update host software components
>  that run the VM, such as QEMU or even the host kernel.  At this time,
> -cpr-reboot is the only available mode.
> +the cpr-reboot and cpr-transfer modes are available.
>  
>  Because QEMU is restarted on the same host, with access to the same
>  local devices, CPR is allowed in certain cases where normal migration
> @@ -53,7 +53,7 @@ RAM is copied to the migration URI.
>  Outgoing:
>    * Set the migration mode parameter to ``cpr-reboot``.
>    * Set the ``x-ignore-shared`` capability if desired.
> -  * Issue the ``migrate`` command.  It is recommended the the URI be a
> +  * Issue the ``migrate`` command.  It is recommended the URI be a
>      ``file`` type, but one can use other types such as ``exec``,
>      provided the command captures all the data from the outgoing side,
>      and provides all the data to the incoming side.
> @@ -145,3 +145,175 @@ Caveats
>  
>  cpr-reboot mode may not be used with postcopy, background-snapshot,
>  or COLO.
> +
> +cpr-transfer mode
> +-----------------
> +
> +This mode allows the user to transfer a guest to a new QEMU instance
> +on the same host with minimal guest pause time, by preserving guest
> +RAM in place, albeit with new virtual addresses in new QEMU.

IMHO it's important to have some words discussing the diff
v.s. ignore-shared, because from above it's undistinguishable from it.

The important bit could be that cpr-transfer allows pinning.  If that's too
internal a concept, we could mention "unlike an ignore-shared migration,
cpr-transfer will start to enable local migrations to work seamlessly with
all kinds of device assignments like VFIO, vDPA and so on".

> +
> +The user starts new QEMU on the same host as old QEMU, with the
> +same arguments as old QEMU, plus the ``-incoming option``.  The user
> +issues the migrate command to old QEMU, which stops the VM, saves
> +state to the migration channels, and enters the postmigrate state.
> +Execution resumes in new QEMU.
> +
> +This mode requires a second migration channel type "cpr" in the
> +channel arguments on the outgoing side.  The channel must be a type,
> +such as unix socket, that supports SCM_RIGHTS.  However, the cpr
> +channel cannot be added to the list of channels for a migrate-incoming
> +command, because it must be read before new QEMU opens a monitor.
> +Instead, the user passes the channel as a second -incoming
> +command-line argument to new QEMU using JSON syntax.
> +
> +Usage
> +^^^^^
> +
> +Memory backend objects must have the ``share=on`` attribute.
> +
> +The VM must be started with the ``-machine aux-ram-share=on``
> +option.  This causes implicit RAM blocks (those not described by
> +a memory-backend object) to be allocated by mmap'ing a memfd.
> +Examples include VGA and ROM.
> +
> +Outgoing:
> +  * Set the migration mode parameter to ``cpr-transfer``.
> +  * Issue the ``migrate`` command, containing a main channel and
> +    a cpr channel.
> +
> +Incoming:
> +  * Start new QEMU with two ``-incoming`` options.
> +  * If the VM was running when the outgoing ``migrate`` command was
> +    issued, then QEMU automatically resumes VM execution.
> +
> +Caveats
> +^^^^^^^
> +
> +cpr-transfer mode may not be used with postcopy, background-snapshot,
> +or COLO.

Maybe we can even remove this line.  It's not like someone could think
about supporting any of above; they just don't apply in cpr context.

> +
> +memory-backend-epc is not supported.
> +
> +The main incoming migration channel cannot be a file type.
> +
> +If the main incoming migration channel is a tcp type, then the port
> +cannot be 0 (meaning dynamically choose a port).
> +
> +When using ``-incoming defer``, you must issue the migrate command to
> +old QEMU before issuing any monitor commands to new QEMU, because new
> +QEMU blocks waiting to read from the cpr channel before starting its
> +monitor, and old QEMU does not write to the channel until the migrate
> +command is issued.  However, new QEMU does not open and read the
> +main migration channel until you issue the migrate incoming command.
> +
> +Example 1: incoming channel
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +In these examples, we simply restart the same version of QEMU, but
> +in a real scenario one would start new QEMU on the incoming side.
> +Note that new QEMU does not print the monitor prompt until old QEMU
> +has issued the migrate command.  The outgoing side uses QMP because
> +HMP cannot specify a CPR channel.  Some QMP responses are omitted for
> +brevity.
> +
> +::
> +
> +  Outgoing:                             Incoming:
> +
> +  # qemu-kvm -qmp stdio
> +  -object memory-backend-file,id=ram0,size=4G,
> +  mem-path=/dev/shm/ram0,share=on -m 4G
> +  -machine aux-ram-share=on
> +  ...
> +                                        # qemu-kvm -monitor stdio
> +                                        -incoming tcp:0:44444
> +                                        -incoming '{"channel-type": "cpr",
> +                                          "addr": { "transport": "socket",
> +                                          "type": "unix", "path": "cpr.sock"}}'
> +                                        ...
> +  {"execute":"qmp_capabilities"}
> +
> +  {"execute": "query-status"}
> +  {"return": {"status": "running",
> +              "running": true}}
> +
> +  {"execute":"migrate-set-parameters",
> +   "arguments":{"mode":"cpr-transfer"}}
> +
> +  {"execute": "migrate", "arguments": { "channels": [
> +    {"channel-type": "main",
> +     "addr": { "transport": "socket", "type": "inet",
> +               "host": "0", "port": "44444" }},
> +    {"channel-type": "cpr",
> +     "addr": { "transport": "socket", "type": "unix",
> +               "path": "cpr.sock" }}]}}
> +
> +                                        QEMU 10.0.50 monitor
> +                                        (qemu) info status
> +                                        VM status: running
> +
> +  {"execute": "query-status"}
> +  {"return": {"status": "postmigrate",
> +              "running": false}}
> +
> +Example 2: incoming defer
> +^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +This example uses ``-incoming defer`` to hot plug a device before
> +accepting the main migration channel.  Again note you must issue the
> +migrate command to old QEMU before you can issue any monitor
> +commands to new QEMU.
> +
> +
> +::
> +
> +  Outgoing:                             Incoming:
> +
> +  # qemu-kvm -monitor stdio
> +  -object memory-backend-file,id=ram0,size=4G,
> +  mem-path=/dev/shm/ram0,share=on -m 4G
> +  -machine aux-ram-share=on
> +  ...
> +                                        # qemu-kvm -monitor stdio
> +                                        -incoming defer
> +                                        -incoming '{"channel-type": "cpr",
> +                                          "addr": { "transport": "socket",
> +                                          "type": "unix", "path": "cpr.sock"}}'
> +                                        ...
> +  {"execute":"qmp_capabilities"}
> +
> +  {"execute": "device_add",
> +   "arguments": {"driver": "pcie-root-port"}}
> +
> +  {"execute":"migrate-set-parameters",
> +   "arguments":{"mode":"cpr-transfer"}}
> +
> +  {"execute": "migrate", "arguments": { "channels": [
> +    {"channel-type": "main",
> +     "addr": { "transport": "socket", "type": "inet",
> +               "host": "0", "port": "44444" }},
> +    {"channel-type": "cpr",
> +     "addr": { "transport": "socket", "type": "unix",
> +               "path": "cpr.sock" }}]}}
> +
> +                                        QEMU 10.0.50 monitor
> +                                        (qemu) info status
> +                                        VM status: paused (inmigrate)
> +                                        (qemu) device_add pcie-root-port
> +                                        (qemu) migrate_incoming tcp:0:44444
> +                                        (qemu) info status
> +                                        VM status: running
> +
> +  {"execute": "query-status"}
> +  {"return": {"status": "postmigrate",
> +              "running": false}}
> +
> +Futures
> +^^^^^^^
> +
> +cpr-transfer mode is based on a capability to transfer open file
> +descriptors from old to new QEMU.  In the future, descriptors for
> +vfio, iommufd, vhost, and char devices could be transferred,
> +preserving those devices and their kernel state without interruption,
> +even if they do not explicitly support live migration.
> -- 
> 1.8.3.1
> 

-- 
Peter Xu