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

[Daniel P . Berrangé]

On Mon, Aug 08, 2022 at 11:41:21AM +0530, Het Gala wrote:
> 
> On 02/08/22 1:23 pm, Markus Armbruster wrote:
> > 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.
> 
> Hi Daniel and other migraton maintainers: Dr. David and Juan, what
> is your opinion on this. And how can we go forward implementing
> this. Some pointers and ideas from your end would be helpful too.

Note, i'm not a migration maintainer, just offering advice from the
sidelines :-)

With regards,
Daniel
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