Re: [PATCH v2 2/7] multifd: modifying 'migrate' qmp command to add multifd socket on particular src and dest pair

[Markus Armbruster]

Het Gala <het.gala@nutanix.com> writes:

> On 26/07/22 4:43 pm, Daniel P. Berrangé wrote:
>> On Thu, Jul 21, 2022 at 07:56:15PM +0000, Het Gala wrote:
>>> i) Modified the format of the qemu monitor command : 'migrate' by adding a 
>>> list,
>>>     each element in the list consisting of multifd connection parameters: 
>>> source
>>>     uri, destination uri and of the number of multifd channels between each 
>>> pair.
>>>
>>> ii) Information of all multifd connection parameters' list and length of the
>>>      list is stored in 'OutgoingMigrateParams' struct.
>>>
>>> Suggested-by: Manish Mishra <manish.mishra@nutanix.com>
>>> Signed-off-by: Het Gala <het.gala@nutanix.com>
>>> ---
>>>   migration/migration.c | 52 +++++++++++++++++++++++++++++--------
>>>   migration/socket.c    | 60 ++++++++++++++++++++++++++++++++++++++++---
>>>   migration/socket.h    | 19 +++++++++++++-
>>>   monitor/hmp-cmds.c    |  1 +
>>>   qapi/migration.json   | 47 +++++++++++++++++++++++++++++----
>>>   5 files changed, 160 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/qapi/migration.json b/qapi/migration.json
>>> index 81185d4311..456247af8f 100644
>>> --- a/qapi/migration.json
>>> +++ b/qapi/migration.json
>>> @@ -1449,12 +1449,37 @@
>>>   ##
>>>   { 'command': 'migrate-continue', 'data': {'state': 'MigrationStatus'} }
>>>   +##
>>> +# @MigrateUriParameter:
>>> +#
>>> +# Information regarding which source interface is connected to which
>>> +# destination interface and number of multifd channels over each interface.
>>> +#
>>> +# @source-uri: uri of the source VM. Default port number is 0.
>>> +#
>>> +# @destination-uri: uri of the destination VM
>>> +#
>>> +# @multifd-channels: number of parallel multifd channels used to migrate 
>>> data
>>> +#                    for specific source-uri and destination-uri. Default 
>>> value
>>> +#                    in this case is 2 (Since 7.1)
>>> +#
>>> +##
>>> +{ 'struct' : 'MigrateUriParameter',
>>> +  'data' : { 'source-uri' : 'str',
>>> +             'destination-uri' : 'str',
>>> +             '*multifd-channels' : 'uint8'} }
>>> +
>>>   ##
>>>   # @migrate:
>>>   #
>>>   # Migrates the current running guest to another Virtual Machine.
>>>   #
>>>   # @uri: the Uniform Resource Identifier of the destination VM
>>> +#       for migration thread
>>> +#
>>> +# @multi-fd-uri-list: list of pair of source and destination VM Uniform
>>> +#                     Resource Identifiers with number of multifd-channels
>>> +#                     for each pair
>>>   #
>>>   # @blk: do block migration (full disk copy)
>>>   #
>>> @@ -1474,20 +1499,32 @@
>>>   # 1. The 'query-migrate' command should be used to check migration's 
>>> progress
>>>   #    and final result (this information is provided by the 'status' 
>>> member)
>>>   #
>>> -# 2. All boolean arguments default to false
>>> +# 2. The uri argument should have the Uniform Resource Identifier of 
>>> default
>>> +#    destination VM. This connection will be bound to default network
>>>   #
>>> -# 3. The user Monitor's "detach" argument is invalid in QMP and should not
>>> +# 3. All boolean arguments default to false
>>> +#
>>> +# 4. The user Monitor's "detach" argument is invalid in QMP and should not
>>>   #    be used
>>>   #
>>>   # Example:
>>>   #
>>> -# -> { "execute": "migrate", "arguments": { "uri": "tcp:0:4446" } }
>>> +# -> { "execute": "migrate",
>>> +#      "arguments": {
>>> +#          "uri": "tcp:0:4446",
>>> +#          "multi-fd-uri-list": [ { "source-uri": "tcp::6900",
>>> +#                                   "destination-uri": "tcp:0:4480",
>>> +#                                   "multifd-channels": 4},
>>> +#                                 { "source-uri": "tcp:10.0.0.0: ",
>>> +#                                   "destination-uri": "tcp:11.0.0.0:7789",
>>> +#                                   "multifd-channels": 5} ] } }
>>>   # <- { "return": {} }
>>>   #
>>>   ##
>>>   { 'command': 'migrate',
>>> -  'data': {'uri': 'str', '*blk': 'bool', '*inc': 'bool',
>>> -           '*detach': 'bool', '*resume': 'bool' } }
>>> +  'data': {'uri': 'str', '*multi-fd-uri-list': ['MigrateUriParameter'],
>>> +           '*blk': 'bool', '*inc': 'bool', '*detach': 'bool',
>>> +           '*resume': 'bool' } }
>>
>> Considering the existing migrate API from a QAPI design POV, I
>> think there are several significant flaws with it
>>
>> The use of URIs is the big red flag. It is basically a data encoding
>> scheme within a data encoding scheme.  QEMU code should be able to
>> directly work with the results from QAPI, without having todo a
>> second level of parsing.

Concur.

>> URIs made sense in the context of HMP or the QemuOpts CLI, but do not
>> make sense in QMP. We made a mistake in this respect when we first
>> introduced QMP and implemented 'migrate'.
>>
>> If we going to extend the migrate API I think we should stop using URIs
>> for the new fields, and instead define a QAPI discriminated union for
>> the different data transport backends we offer.
>>
>>       { 'enum': 'MigrateTransport',
>>         'data': ['socket', 'exec'] }
>>
>>       { 'union': 'MigrateAddress',
>>         'base': { 'transport': 'MigrateTransport'},
>>         'discriminator': 'transport',
>>         'data': {
>>             'socket': 'SocketAddress',
>>         'exec': ['str'],
>>         }
>>
>> NB, 'socket' should be able to cover all of  'tcp', 'unix', 'vsock'
>> and 'fd' already. I'm fuzzy on best way to represent RDMA.
>>
>>
>> IIUC, the desire of migration maintainers is that we can ultimately
>> have multifd as the preferred, or even only, mechanism. Aside from
>> the main outbound migration control channel, and the multifd
>> data channels, IIUC we have a potential desire to have more channels
>> for post-copy async requests.
>>
>> This all suggests to me a more general representation along the
>> lines of:
>>
>>    { 'enum': 'MigrateChannelType',
>>      'data': ['control', 'data', 'async'] }
>>
>>    { 'struct': 'MigrateChannel',
>>      'data': {
>>         'type': 'MigrateChannelType',
>>         'src-addr': 'MigrateAddress',
>>         'dst-addr': 'MigrateAddress',
>>         'count': 'int',
>>      } }
>>
>>    { 'comand': 'migrate',
>>      'data': {
>>        '*uri': 'str'
>>        '*channels': ['MigrateChannel']
>>      }
>>    }
>>
>> With 'uri' and 'channels' being mutually exclusive here.
>>
>> This whole scheme brings in redundancy wrt to the 'migrate-set-parameters'
>> API wrt multifd - essentally the same data is now being set in two
>> different places. IMHO, we should declare the 'multifd' capability
>> and the 'multifd-chanels' parameter deprecated, since the information
>> they provide is totally redundant, if you're giving an explicit list
>> of channels to 'migrate'.
>
> Hi Daniel. Initially while brainstorming this idea for the first time, we 
> also came up with the same thought of depricating the migrate 
> API. But how will we achieve this now and how is it going to work. Is it like 
> we will be making migate V2 APIs initially, integrate it and then 
> depricate the old one? would be happy to get some pointers from your end.

Migration maintainers, please advise.