Re: [PATCH v2 2/6] migration: Updated QAPI format for 'migrate' qemu monitor command

[Daniel P . Berrangé]

On Wed, Feb 08, 2023 at 09:35:56AM +0000, Het Gala wrote:
> Existing 'migrate' QAPI design enforces transport mechanism, ip address
> of destination interface and corresponding port number in the form
> of a unified string 'uri' parameter for initiating a migration stream.
> This scheme has a significant flaw in it - double encoding of existing
> URIs to extract migration info.
> 
> The current patch maps QAPI uri design onto well defined MigrateChannel
> struct. This modified QAPI helps in preventing multi-level uri
> encodings ('uri' parameter is kept for backward compatibility).
> 
> Suggested-by: Daniel P. Berrange <berrange@redhat.com>
> Suggested-by: Manish Mishra <manish.mishra@nutanix.com>
> Suggested-by: Aravind Retnakaran <aravind.retnakaran@nutanix.com>
> Signed-off-by: Het Gala <het.gala@nutanix.com>
> ---
>  qapi/migration.json | 131 +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 129 insertions(+), 2 deletions(-)
> 
> diff --git a/qapi/migration.json b/qapi/migration.json
> index c84fa10e86..79acfcfe4e 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1449,12 +1449,108 @@
>  ##
>  { 'command': 'migrate-continue', 'data': {'state': 'MigrationStatus'} }
>  
> +##
> +# @MigrateTransport:
> +#
> +# The supported communication transport mechanisms for migration
> +#
> +# @socket: Supported communication type between two devices for migration.
> +#          Socket is able to cover all of 'tcp', 'unix', 'vsock' and
> +#          'fd' already
> +#
> +# @exec: Supported communication type to redirect migration stream into file.
> +#
> +# @rdma: Supported communication type to redirect rdma type migration stream.
> +#
> +# Since 8.0
> +##
> +{ 'enum': 'MigrateTransport',
> +  'data': ['socket', 'exec', 'rdma'] }
> +
> +##
> +# @MigrateSocketAddr:
> +#
> +# To support different type of socket.
> +#
> +# @socket-type: Different type of socket connections.
> +#
> +# Since 8.0
> +##
> +{ 'struct': 'MigrateSocketAddr',
> +  'data': {'socket-type': 'SocketAddress' } }

I'd really like this struct to go away, but if it must exist,
then call this field 'addr', as I think 'socket-type' is overly
verbose.

> +
> +##
> +# @MigrateExecAddr:
> + #
> + # Since 8.0
> + ##
> +{ 'struct': 'MigrateExecAddr',
> +   'data' : {'data': ['str'] } }

Instead of having the field called 'data' lets me more
descriptive, and perhaps rename the struct too:

 { 'struct': 'MigrateCommand',
    'data' : {'args': ['str'] } }

Any thoughts on whether we should allow for setting env varibles
too ?

> +##
> +# @MigrateRdmaAddr:
> +#
> +# Since 8.0
> +##
> +{ 'struct': 'MigrateRdmaAddr',
> +   'data' : {'data': 'InetSocketAddress' } }

InetSocketAddress is a plain struct, so I think we can use
that directly, no ?

> +
> +##
> +# @MigrateAddress:
> +#
> +# The options available for communication transport mechanisms for migration
> +#
> +# Since 8.0
> +##
> +{ 'union' : 'MigrateAddress',
> +  'base' : { 'transport' : 'MigrateTransport'},
> +  'discriminator' : 'transport',
> +  'data' : {
> +    'socket' : 'MigrateSocketAddr',
> +    'exec' : 'MigrateExecAddr',
> +    'rdma': 'MigrateRdmaAddr' } }

Ideally this would be

   'data' : {
     'socket' : 'SocketAddress',
     'exec' : 'MigrateCommand',
     'rdma': 'InetSocketAddress' } }

though the first SocketAddress isn't possible unless it is easy to
lift the QAPI limitation.


>  ##
>  # @migrate:
>  #
>  # Migrates the current running guest to another Virtual Machine.
>  #
>  # @uri: the Uniform Resource Identifier of the destination VM
> +#       for migration thread
> +#
> +# @channel: Struct containing migration channel type, along with all
> +#           the details of destination interface required for initiating
> +#           a migration stream.
>  #
>  # @blk: do block migration (full disk copy)
>  #
> @@ -1479,15 +1575,46 @@
>  # 3. The user Monitor's "detach" argument is invalid in QMP and should not
>  #    be used
>  #
> +# 4. The uri argument should have the Uniform Resource Identifier of default
> +#    destination VM. This connection will be bound to default network
> +#
> +# 5. Both 'uri' and 'channel' arguments, are mutually exclusive but, at least
> +#    one of the two arguments should be present.
> +#
>  # Example:
>  #
>  # -> { "execute": "migrate", "arguments": { "uri": "tcp:0:4446" } }
>  # <- { "return": {} }
>  #
> +# -> { "execute": "migrate",
> +#      "arguments": {
> +#          "channel": { "channeltype": "main",
> +#                        "addr": { "transport": "socket",
> +#                                  "socket-type": { "type': "inet',
> +#                                                   "host": "10.12.34.9",
> +#                                                   "port": "1050" } } } } }
> +# <- { "return": {} }
> +#
> +# -> { "execute": "migrate",
> +#      "arguments": {
> +#          "channel": { "channeltype": "main",
> +#                       "addr": { "transport": "exec",
> +#                                 "exec": ["/bin/nc", "-U",
> +#                                          "/some/sock" ] } } } }
> +# <- { "return": {} }
> +#
> +# -> { "execute": "migrate",
> +#      "arguments": {
> +#          "channel": { "channeltype": "main",
> +#                       "addr": { "transport": "rdma",
> +#                                 "rdma": { "host": "10.12.34.9",
> +#                                           "port": "1050" } } } } }
> +# <- { "return": {} }
> +#
>  ##
>  { 'command': 'migrate',
> -  'data': {'uri': 'str', '*blk': 'bool', '*inc': 'bool',
> -           '*detach': 'bool', '*resume': 'bool' } }
> +  'data': {'*uri': 'str', '*channel': 'MigrateChannel', '*blk': 'bool',
> +           '*inc': 'bool', '*detach': 'bool', '*resume': 'bool' } }

IIRC, the intention was to allow multiple channels to be set in a
follow up to this series. If so that would require adding yet another
field as an array of MigrateChannel.  Should we just go for the
array straight away, and just limit it to 1 element  as a runtime
check ? eg

  'data': {'*uri': 'str', '*channels': ['MigrateChannel'], '*blk': 'bool',
           '*inc': 'bool', '*detach': 'bool', '*resume': 'bool' } }


With regards,
Daniel
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