Re: [RFC PATCH 05/13] migration: Reduce a bit of duplication in migration.json

[Markus Armbruster <armbru@redhat.com>]

Fabiano Rosas <farosas@suse.de> writes:

> Introduce a new MigrationConfigBase, to allow most of the duplication
> in migration.json to be eliminated.
>
> The reason we need MigrationParameters and MigrationSetParameters is
> that the internal parameter representation in the migration code, as
> well as the user-facing return of query-migrate-parameters use one
> type for the TLS options (tls-creds, tls-hostname, tls-authz), while
> the user-facing input from migrate-set-parameters uses another.
>
> The difference is in whether the NULL values is accepted. The former
> considers NULL as invalid, while the latter doesn't.
>
> Move all other (non-TLS) options into the new type and make it a base
> type for the two diverging types so that each child type can declare
> the TLS options in its own way.
>
> Nothing changes in the user API, nothing changes in the internal
> representation, but we save several lines of duplication in
> migration.json.

I double-checked the external interface.  There's a new member @tls, but
you already told us it's an accident.

Documentation does change.  More on that below.

> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  qapi/migration.json | 358 +++++++++++++-------------------------------
>  1 file changed, 108 insertions(+), 250 deletions(-)
>
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 8b9c53595c..5a4d5a2d3e 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -914,202 +914,6 @@
>             'zero-page-detection',
>             'direct-io'] }
>  
> -##
> -# @MigrateSetParameters:
> -#
> -# @announce-initial: Initial delay (in milliseconds) before sending
> -#     the first announce (Since 4.0)
> -#
> -# @announce-max: Maximum delay (in milliseconds) between packets in
> -#     the announcement (Since 4.0)
> -#
> -# @announce-rounds: Number of self-announce packets sent after
> -#     migration (Since 4.0)
> -#
> -# @announce-step: Increase in delay (in milliseconds) between
> -#     subsequent packets in the announcement (Since 4.0)
> -#
> -# @throttle-trigger-threshold: The ratio of bytes_dirty_period and
> -#     bytes_xfer_period to trigger throttling.  It is expressed as
> -#     percentage.  The default value is 50.  (Since 5.0)
> -#
> -# @cpu-throttle-initial: Initial percentage of time guest cpus are
> -#     throttled when migration auto-converge is activated.  The
> -#     default value is 20.  (Since 2.7)
> -#
> -# @cpu-throttle-increment: throttle percentage increase each time
> -#     auto-converge detects that migration is not making progress.
> -#     The default value is 10.  (Since 2.7)
> -#
> -# @cpu-throttle-tailslow: Make CPU throttling slower at tail stage At
> -#     the tail stage of throttling, the Guest is very sensitive to CPU
> -#     percentage while the @cpu-throttle -increment is excessive
> -#     usually at tail stage.  If this parameter is true, we will
> -#     compute the ideal CPU percentage used by the Guest, which may
> -#     exactly make the dirty rate match the dirty rate threshold.
> -#     Then we will choose a smaller throttle increment between the one
> -#     specified by @cpu-throttle-increment and the one generated by
> -#     ideal CPU percentage.  Therefore, it is compatible to
> -#     traditional throttling, meanwhile the throttle increment won't
> -#     be excessive at tail stage.  The default value is false.  (Since
> -#     5.1)
> -#
> -# @tls-creds: ID of the 'tls-creds' object that provides credentials
> -#     for establishing a TLS connection over the migration data
> -#     channel.  On the outgoing side of the migration, the credentials
> -#     must be for a 'client' endpoint, while for the incoming side the
> -#     credentials must be for a 'server' endpoint.  Setting this to a
> -#     non-empty string enables TLS for all migrations.  An empty
> -#     string means that QEMU will use plain text mode for migration,
> -#     rather than TLS.  This is the default.  (Since 2.7)
> -#
> -# @tls-hostname: migration target's hostname for validating the
> -#     server's x509 certificate identity.  If empty, QEMU will use the
> -#     hostname from the migration URI, if any.  A non-empty value is
> -#     required when using x509 based TLS credentials and the migration
> -#     URI does not include a hostname, such as fd: or exec: based
> -#     migration.  (Since 2.7)
> -#
> -#     Note: empty value works only since 2.9.
> -#
> -# @tls-authz: ID of the 'authz' object subclass that provides access
> -#     control checking of the TLS x509 certificate distinguished name.
> -#     This object is only resolved at time of use, so can be deleted
> -#     and recreated on the fly while the migration server is active.
> -#     If missing, it will default to denying access (Since 4.0)
> -#
> -# @max-bandwidth: maximum speed for migration, in bytes per second.
> -#     (Since 2.8)
> -#
> -# @avail-switchover-bandwidth: to set the available bandwidth that
> -#     migration can use during switchover phase.  NOTE!  This does not
> -#     limit the bandwidth during switchover, but only for calculations
> -#     when making decisions to switchover.  By default, this value is
> -#     zero, which means QEMU will estimate the bandwidth
> -#     automatically.  This can be set when the estimated value is not
> -#     accurate, while the user is able to guarantee such bandwidth is
> -#     available when switching over.  When specified correctly, this
> -#     can make the switchover decision much more accurate.
> -#     (Since 8.2)
> -#
> -# @downtime-limit: set maximum tolerated downtime for migration.
> -#     maximum downtime in milliseconds (Since 2.8)
> -#
> -# @x-checkpoint-delay: The delay time (in ms) between two COLO
> -#     checkpoints in periodic mode.  (Since 2.8)
> -#
> -# @multifd-channels: Number of channels used to migrate data in
> -#     parallel.  This is the same number that the number of sockets
> -#     used for migration.  The default value is 2 (since 4.0)
> -#
> -# @xbzrle-cache-size: cache size to be used by XBZRLE migration.  It
> -#     needs to be a multiple of the target page size and a power of 2
> -#     (Since 2.11)
> -#
> -# @max-postcopy-bandwidth: Background transfer bandwidth during
> -#     postcopy.  Defaults to 0 (unlimited).  In bytes per second.
> -#     (Since 3.0)
> -#
> -# @max-cpu-throttle: maximum cpu throttle percentage.  Defaults to 99.
> -#     (Since 3.1)
> -#
> -# @multifd-compression: Which compression method to use.  Defaults to
> -#     none.  (Since 5.0)
> -#
> -# @multifd-zlib-level: Set the compression level to be used in live
> -#     migration, the compression level is an integer between 0 and 9,
> -#     where 0 means no compression, 1 means the best compression
> -#     speed, and 9 means best compression ratio which will consume
> -#     more CPU.  Defaults to 1.  (Since 5.0)
> -#
> -# @multifd-qatzip-level: Set the compression level to be used in live
> -#     migration. The level is an integer between 1 and 9, where 1 means
> -#     the best compression speed, and 9 means the best compression
> -#     ratio which will consume more CPU. Defaults to 1.  (Since 9.2)
> -#
> -# @multifd-zstd-level: Set the compression level to be used in live
> -#     migration, the compression level is an integer between 0 and 20,
> -#     where 0 means no compression, 1 means the best compression
> -#     speed, and 20 means best compression ratio which will consume
> -#     more CPU.  Defaults to 1.  (Since 5.0)
> -#
> -# @block-bitmap-mapping: Maps block nodes and bitmaps on them to
> -#     aliases for the purpose of dirty bitmap migration.  Such aliases
> -#     may for example be the corresponding names on the opposite site.
> -#     The mapping must be one-to-one, but not necessarily complete: On
> -#     the source, unmapped bitmaps and all bitmaps on unmapped nodes
> -#     will be ignored.  On the destination, encountering an unmapped
> -#     alias in the incoming migration stream will result in a report,
> -#     and all further bitmap migration data will then be discarded.
> -#     Note that the destination does not know about bitmaps it does
> -#     not receive, so there is no limitation or requirement regarding
> -#     the number of bitmaps received, or how they are named, or on
> -#     which nodes they are placed.  By default (when this parameter
> -#     has never been set), bitmap names are mapped to themselves.
> -#     Nodes are mapped to their block device name if there is one, and
> -#     to their node name otherwise.  (Since 5.2)
> -#
> -# @x-vcpu-dirty-limit-period: Periodic time (in milliseconds) of dirty
> -#     limit during live migration.  Should be in the range 1 to
> -#     1000ms.  Defaults to 1000ms.  (Since 8.1)
> -#
> -# @vcpu-dirty-limit: Dirtyrate limit (MB/s) during live migration.
> -#     Defaults to 1.  (Since 8.1)
> -#
> -# @mode: Migration mode.  See description in @MigMode.  Default is
> -#     'normal'.  (Since 8.2)
> -#
> -# @zero-page-detection: Whether and how to detect zero pages.
> -#     See description in @ZeroPageDetection.  Default is 'multifd'.
> -#     (since 9.0)
> -#
> -# @direct-io: Open migration files with O_DIRECT when possible.  This
> -#     only has effect if the @mapped-ram capability is enabled.
> -#     (Since 9.1)
> -#
> -# Features:
> -#
> -# @unstable: Members @x-checkpoint-delay and
> -#     @x-vcpu-dirty-limit-period are experimental.
> -#
> -# TODO: either fuse back into MigrationParameters, or make
> -#     MigrationParameters members mandatory
> -#
> -# Since: 2.4
> -##
> -{ 'struct': 'MigrateSetParameters',
> -  'data': { '*announce-initial': 'size',
> -            '*announce-max': 'size',
> -            '*announce-rounds': 'size',
> -            '*announce-step': 'size',
> -            '*throttle-trigger-threshold': 'uint8',
> -            '*cpu-throttle-initial': 'uint8',
> -            '*cpu-throttle-increment': 'uint8',
> -            '*cpu-throttle-tailslow': 'bool',
> -            '*tls-creds': 'StrOrNull',
> -            '*tls-hostname': 'StrOrNull',
> -            '*tls-authz': 'StrOrNull',
> -            '*max-bandwidth': 'size',
> -            '*avail-switchover-bandwidth': 'size',
> -            '*downtime-limit': 'uint64',
> -            '*x-checkpoint-delay': { 'type': 'uint32',
> -                                     'features': [ 'unstable' ] },
> -            '*multifd-channels': 'uint8',
> -            '*xbzrle-cache-size': 'size',
> -            '*max-postcopy-bandwidth': 'size',
> -            '*max-cpu-throttle': 'uint8',
> -            '*multifd-compression': 'MultiFDCompression',
> -            '*multifd-zlib-level': 'uint8',
> -            '*multifd-qatzip-level': 'uint8',
> -            '*multifd-zstd-level': 'uint8',
> -            '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ],
> -            '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
> -                                            'features': [ 'unstable' ] },
> -            '*vcpu-dirty-limit': 'uint64',
> -            '*mode': 'MigMode',
> -            '*zero-page-detection': 'ZeroPageDetection',
> -            '*direct-io': 'bool' } }
> -
>  ##
>  # @migrate-set-parameters:
>  #
> @@ -1127,9 +931,7 @@
>    'data': 'MigrateSetParameters' }
>  
>  ##
> -# @MigrationParameters:
> -#
> -# The optional members aren't actually optional.
> +# @MigrationConfigBase:
>  #
>  # @announce-initial: Initial delay (in milliseconds) before sending
>  #     the first announce (Since 4.0)
> @@ -1168,26 +970,6 @@
>  #     be excessive at tail stage.  The default value is false.  (Since
>  #     5.1)
>  #
> -# @tls-creds: ID of the 'tls-creds' object that provides credentials
> -#     for establishing a TLS connection over the migration data
> -#     channel.  On the outgoing side of the migration, the credentials
> -#     must be for a 'client' endpoint, while for the incoming side the
> -#     credentials must be for a 'server' endpoint.  An empty string
> -#     means that QEMU will use plain text mode for migration, rather
> -#     than TLS.  (Since 2.7)
> -#
> -#     Note: 2.8 omits empty @tls-creds instead.
> -#
> -# @tls-hostname: migration target's hostname for validating the
> -#     server's x509 certificate identity.  If empty, QEMU will use the
> -#     hostname from the migration URI, if any.  (Since 2.7)
> -#
> -#     Note: 2.8 omits empty @tls-hostname instead.
> -#
> -# @tls-authz: ID of the 'authz' object subclass that provides access
> -#     control checking of the TLS x509 certificate distinguished name.
> -#     (Since 4.0)
> -#
>  # @max-bandwidth: maximum speed for migration, in bytes per second.
>  #     (Since 2.8)
>  #
> @@ -1277,45 +1059,121 @@
>  #     only has effect if the @mapped-ram capability is enabled.
>  #     (Since 9.1)
>  #
> +# @tls: Whether to use TLS. If this is set the options @tls-authz,
> +#     @tls-creds, @tls-hostname are mandatory and a valid string is
> +#     expected. (Since 10.1)
> +#
>  # Features:
>  #
>  # @unstable: Members @x-checkpoint-delay and
>  #     @x-vcpu-dirty-limit-period are experimental.
>  #
> +# Since: 10.1
> +##
> +{ 'struct': 'MigrationConfigBase',
> +  'data': { '*announce-initial': 'size',
> +            '*announce-max': 'size',
> +            '*announce-rounds': 'size',
> +            '*announce-step': 'size',
> +            '*throttle-trigger-threshold': 'uint8',
> +            '*cpu-throttle-initial': 'uint8',
> +            '*cpu-throttle-increment': 'uint8',
> +            '*cpu-throttle-tailslow': 'bool',
> +            '*max-bandwidth': 'size',
> +            '*avail-switchover-bandwidth': 'size',
> +            '*downtime-limit': 'uint64',
> +            '*x-checkpoint-delay': { 'type': 'uint32',
> +                                     'features': [ 'unstable' ] },
> +            '*multifd-channels': 'uint8',
> +            '*xbzrle-cache-size': 'size',
> +            '*max-postcopy-bandwidth': 'size',
> +            '*max-cpu-throttle': 'uint8',
> +            '*multifd-compression': 'MultiFDCompression',
> +            '*multifd-zlib-level': 'uint8',
> +            '*multifd-qatzip-level': 'uint8',
> +            '*multifd-zstd-level': 'uint8',
> +            '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ],
> +            '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
> +                                            'features': [ 'unstable' ] },
> +            '*vcpu-dirty-limit': 'uint64',
> +            '*mode': 'MigMode',
> +            '*zero-page-detection': 'ZeroPageDetection',
> +            '*direct-io': 'bool',
> +            '*tls': 'bool' } }
> +
> +##
> +# @MigrationParameters:
> +#
> +# The optional members of the base type aren't actually optional.
> +#
> +# @tls-creds: ID of the 'tls-creds' object that provides credentials
> +#     for establishing a TLS connection over the migration data
> +#     channel.  On the outgoing side of the migration, the credentials
> +#     must be for a 'client' endpoint, while for the incoming side the
> +#     credentials must be for a 'server' endpoint.  Setting this to a
> +#     non-empty string enables TLS for all migrations.  An empty
> +#     string means that QEMU will use plain text mode for migration,
> +#     rather than TLS.  (Since 2.7)
> +#
> +# @tls-hostname: migration target's hostname for validating the
> +#     server's x509 certificate identity.  If empty, QEMU will use the
> +#     hostname from the migration URI, if any.  A non-empty value is
> +#     required when using x509 based TLS credentials and the migration
> +#     URI does not include a hostname, such as fd: or exec: based
> +#     migration.  (Since 2.7)
> +#
> +#     Note: empty value works only since 2.9.
> +#
> +# @tls-authz: ID of the 'authz' object subclass that provides access
> +#     control checking of the TLS x509 certificate distinguished name.
> +#     This object is only resolved at time of use, so can be deleted
> +#     and recreated on the fly while the migration server is active.
> +#     If missing, it will default to denying access (Since 4.0)
> +#
>  # Since: 2.4
>  ##
>  { 'struct': 'MigrationParameters',
> -  'data': { '*announce-initial': 'size',
> -            '*announce-max': 'size',
> -            '*announce-rounds': 'size',
> -            '*announce-step': 'size',
> -            '*throttle-trigger-threshold': 'uint8',
> -            '*cpu-throttle-initial': 'uint8',
> -            '*cpu-throttle-increment': 'uint8',
> -            '*cpu-throttle-tailslow': 'bool',
> -            '*tls-creds': 'str',
> -            '*tls-hostname': 'str',
> -            '*tls-authz': 'str',
> -            '*max-bandwidth': 'size',
> -            '*avail-switchover-bandwidth': 'size',
> -            '*downtime-limit': 'uint64',
> -            '*x-checkpoint-delay': { 'type': 'uint32',
> -                                     'features': [ 'unstable' ] },
> -            '*multifd-channels': 'uint8',
> -            '*xbzrle-cache-size': 'size',
> -            '*max-postcopy-bandwidth': 'size',
> -            '*max-cpu-throttle': 'uint8',
> -            '*multifd-compression': 'MultiFDCompression',
> -            '*multifd-zlib-level': 'uint8',
> -            '*multifd-qatzip-level': 'uint8',
> -            '*multifd-zstd-level': 'uint8',
> -            '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ],
> -            '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
> -                                            'features': [ 'unstable' ] },
> -            '*vcpu-dirty-limit': 'uint64',
> -            '*mode': 'MigMode',
> -            '*zero-page-detection': 'ZeroPageDetection',
> -            '*direct-io': 'bool' } }
> +  'base': 'MigrationConfigBase',
> +  'data': { 'tls-creds': 'str',
> +            'tls-hostname': 'str',
> +            'tls-authz': 'str' } }
> +
> +##
> +# @MigrateSetParameters:
> +#
> +# Compatibility layer to accept null values for the TLS options.
> +#
> +# @tls-creds: ID of the 'tls-creds' object that provides credentials
> +#     for establishing a TLS connection over the migration data
> +#     channel.  On the outgoing side of the migration, the credentials
> +#     must be for a 'client' endpoint, while for the incoming side the
> +#     credentials must be for a 'server' endpoint.  Setting this to a
> +#     non-empty string enables TLS for all migrations.  An empty
> +#     string means that QEMU will use plain text mode for migration,
> +#     rather than TLS.  This is the default.  (Since 2.7)
> +#
> +# @tls-hostname: migration target's hostname for validating the
> +#     server's x509 certificate identity.  If empty, QEMU will use the
> +#     hostname from the migration URI, if any.  A non-empty value is
> +#     required when using x509 based TLS credentials and the migration
> +#     URI does not include a hostname, such as fd: or exec: based
> +#     migration.  (Since 2.7)
> +#
> +#     Note: empty value works only since 2.9.
> +#
> +# @tls-authz: ID of the 'authz' object subclass that provides access
> +#     control checking of the TLS x509 certificate distinguished name.
> +#     This object is only resolved at time of use, so can be deleted
> +#     and recreated on the fly while the migration server is active.
> +#     If missing, it will default to denying access (Since 4.0)
> +#
> +# Since: 2.4
> +##
> +{ 'struct': 'MigrateSetParameters',
> +  'base': 'MigrationConfigBase',
> +  'data': { '*tls-creds': 'StrOrNull',
> +            '*tls-hostname': 'StrOrNull',
> +            '*tls-authz': 'StrOrNull' } }
>  
>  ##
>  # @query-migrate-parameters:

Your change is fairly simple, but that's not obvious from the diff.

1. Move everything but the TLS stuff from MigrationParameters to its new
   base type MigrationConfigBase.

   No change to the interface, obviously.

   Introspection hides the base type, and shows members of
   MigrationParameters in a different order, but order doesn't matter.

   The doc generator isn't smart enough (yet) to hide the base type.
   Apart from that, documentation is unchanged.

2. Replace everything but the TLS stuff from MigrateSetParameters by
   base type MigrateSetParameters.

   Because the base type's members are identical to the members it
   replaces, no change to the interface.

   Introspection hides the base type, and shows members of
   MigrateSetParameters in a different order, but order doesn't matter.

   The base type's member documentation is *not* identical to the member
   documentation it replaces.  To see the differences, I ran
   sphinx-build with -b text before and after, and fed its output to
   diff:

       * **cpu-throttle-initial** ("int", *optional*) -- Initial
         percentage of time guest cpus are throttled when migration
-        auto-converge is activated.  The default value is 20.  (Since
-        2.7)
+        auto-converge is activated.  (Since 2.7)

   We no longer document the default value.  This is wrong.

   The difference before the patch makes some sense.  The members of
   MigrateSetParameters are actually optional command arguments, and
   their defaults must be documented.  The members of
   MigrationParameters aren't actually optional, and talking about
   defaults is misleading there.  We do it anyway in a few places.

   Factoring out MigrationConfigBase leads to a conflict: in its role as
   base of MigrationParameters, it shouldn't talk about defaults, and in
   its role as base of MigrateSetParameters, it should document the
   defaults supplied by migrate-set-parameters.

   There is no perfectly satisfactory solution with our current
   infrastructure.

   We can elect to ignore the problem for now.  Point it out in a TODO
   comment.

   We can try to explain in the doc text that default values apply only
   to migrate-set-parameters.  I expect this to be awkward.  More
   awkward once the doc generator inlines stuff.

       * **cpu-throttle-increment** ("int", *optional*) -- throttle
         percentage increase each time auto-converge detects that
-        migration is not making progress. The default value is 10.
-        (Since 2.7)
+        migration is not making progress. (Since 2.7)
 
    Likewise.
 
-      * **x-checkpoint-delay** ("int", *optional*) -- The delay time
-        (in ms) between two COLO checkpoints in periodic mode.  (Since
-        2.8)
+      * **x-checkpoint-delay** ("int", *optional*) -- the delay time
+        between two COLO checkpoints. (Since 2.8)
 
    Inconsistent before the patch.  The deleted version is better.  Easy
    enough to fix.
 
+      * **tls** ("boolean", *optional*) -- Whether to use TLS. If this
+        is set the options "tls-authz", "tls-creds", "tls-hostname"
+        are mandatory and a valid string is expected. (Since 10.1)
+

    Accident.