Re: [PATCH v6 16/19] qapi: add interface for backend-transfer virtio-net/tap migration

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> To migrate virtio-net TAP device backend (including open fds) locally,
> user should simply set migration parameter
>
>    backend-transfer = ["virtio-net-tap"]
>
> Why not simple boolean? To simplify migration to further versions,
> when more devices will support backend-transfer migration.
>
> Alternatively, we may add per-device option to disable backend-transfer
> migration, but still:
>
> 1. It's more comfortable to set same capabilities/parameters on both
> source and target QEMU, than care about each device.
>
> 2. To not break the design, that machine-type + device options +
> migration capabilities and parameters are fully define the resulting
> migration stream. We'll break this if add in future more
> backend-transfer support in devices under same backend-transfer=true
> parameter.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
>  include/qapi/util.h | 17 ++++++++++++++++
>  migration/options.c | 32 ++++++++++++++++++++++++++++++
>  migration/options.h |  2 ++
>  qapi/migration.json | 47 ++++++++++++++++++++++++++++++++++++---------
>  4 files changed, 89 insertions(+), 9 deletions(-)
>
> diff --git a/include/qapi/util.h b/include/qapi/util.h
> index 29bc4eb865..b953402416 100644
> --- a/include/qapi/util.h
> +++ b/include/qapi/util.h
> @@ -69,4 +69,21 @@ int parse_qapi_name(const char *name, bool complete);
>          _len;                                                       \
>      })
>  
> +/*
> + * For any GenericList @list, return true if it contains specified
> + * element.
> + */
> +#define QAPI_LIST_CONTAINS(list, el)                                \
> +    ({                                                              \
> +        bool _found = false;                                        \
> +        typeof_strip_qual(list) _tail;                              \
> +        for (_tail = list; _tail != NULL; _tail = _tail->next) {    \
> +            if (_tail->value == el) {                               \
> +                _found = true;                                      \
> +                break;                                              \
> +            }                                                       \
> +        }                                                           \
> +        _found;                                                     \
> +    })
> +

Not a fan of lengthy macros.

There's a single use below: migrate_virtio_net_tap().  I can't see
potential uses for such a search in existing code.

Open-code it in migrate_virtio_net_tap()?

>  #endif
> diff --git a/migration/options.c b/migration/options.c
> index 4e923a2e07..137ca2147e 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -13,6 +13,7 @@
>  
>  #include "qemu/osdep.h"
>  #include "qemu/error-report.h"
> +#include "qapi/util.h"
>  #include "exec/target_page.h"
>  #include "qapi/clone-visitor.h"
>  #include "qapi/error.h"
> @@ -262,6 +263,14 @@ bool migrate_mapped_ram(void)
>      return s->capabilities[MIGRATION_CAPABILITY_MAPPED_RAM];
>  }
>  
> +bool migrate_virtio_net_tap(void)
> +{
> +    MigrationState *s = migrate_get_current();
> +
> +    return QAPI_LIST_CONTAINS(s->parameters.backend_transfer,
> +                              BACKEND_TRANSFER_VIRTIO_NET_TAP);
> +}
> +
>  bool migrate_ignore_shared(void)
>  {
>      MigrationState *s = migrate_get_current();
> @@ -960,6 +969,12 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
>      params->has_direct_io = true;
>      params->direct_io = s->parameters.direct_io;
>  
> +    if (s->parameters.backend_transfer) {
> +        params->has_backend_transfer = true;
> +        params->backend_transfer = QAPI_CLONE(BackendTransferList,
> +                                              s->parameters.backend_transfer);
> +    }
> +
>      return params;
>  }
>  
> @@ -993,6 +1008,7 @@ void migrate_params_init(MigrationParameters *params)
>      params->has_mode = true;
>      params->has_zero_page_detection = true;
>      params->has_direct_io = true;
> +    params->has_backend_transfer = true;
>  }
>  
>  /*
> @@ -1179,6 +1195,11 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)
>          return false;
>      }
>  
> +    if (params->has_backend_transfer) {
> +        error_setg(errp, "Not implemented");
> +        return false;
> +    }
> +

This goes away in the next patch.  Fine, but mentioning the gap in the
commit message can save reviewer a bit of work.

>      return true;
>  }
>  
> @@ -1297,6 +1318,10 @@ static void migrate_params_test_apply(MigrateSetParameters *params,
>      if (params->has_direct_io) {
>          dest->direct_io = params->direct_io;
>      }
> +
> +    if (params->has_backend_transfer) {
> +        dest->backend_transfer = params->backend_transfer;
> +    }
>  }
>  
>  static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
> @@ -1429,6 +1454,13 @@ static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
>      if (params->has_direct_io) {
>          s->parameters.direct_io = params->direct_io;
>      }
> +
> +    if (params->has_backend_transfer) {
> +        qapi_free_BackendTransferList(s->parameters.backend_transfer);
> +
> +        s->parameters.backend_transfer = QAPI_CLONE(BackendTransferList,
> +                                                    params->backend_transfer);
> +    }
>  }
>  
>  void qmp_migrate_set_parameters(MigrateSetParameters *params, Error **errp)
> diff --git a/migration/options.h b/migration/options.h
> index 82d839709e..55c0345433 100644
> --- a/migration/options.h
> +++ b/migration/options.h
> @@ -87,6 +87,8 @@ const char *migrate_tls_hostname(void);
>  uint64_t migrate_xbzrle_cache_size(void);
>  ZeroPageDetection migrate_zero_page_detection(void);
>  
> +bool migrate_virtio_net_tap(void);
> +
>  /* parameters helpers */
>  
>  bool migrate_params_check(MigrationParameters *params, Error **errp);
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 2387c21e9c..e39785dc07 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -747,6 +747,18 @@
>        '*transform': 'BitmapMigrationBitmapAliasTransform'
>    } }
>  
> +##
> +# @BackendTransfer:
> +#
> +# @virtio-net-tap: Enable backend-transfer migration for virtio-net/tap. When
> +#     enabled, TAP fds and all related state is passed to target QEMU through
> +#     migration channel (which should be unix socket).

Suggest "are passed to the destination in the migration channel" and
"should be a UNIX domain socket".

docs/devel/qapi-code-gen.rst:

    For legibility, wrap text paragraphs so every line is at most 70
    characters long.

    Separate sentences with two spaces.

> +#
> +# Since: 10.2
> +##
> +{ 'enum': 'BackendTransfer',
> +  'data': [ 'virtio-net-tap' ] }
> +
>  ##
>  # @BitmapMigrationNodeAlias:
>  #
> @@ -924,10 +936,14 @@
>  #     only has effect if the @mapped-ram capability is enabled.
>  #     (Since 9.1)
>  #
> +# @backend-transfer: List of targets to enable backend-transfer
> +#     migration for. This requires migration channel to be a unix
> +#     socket (to pass fds through). (Since 10.2)

Elsewhere, we describe the same restriction like this:

                                          This CPR channel must support
   #     file descriptor transfer with SCM_RIGHTS, i.e. it must be a
   #     UNIX domain socket.

> +#
>  # Features:
>  #
> -# @unstable: Members @x-checkpoint-delay and
> -#     @x-vcpu-dirty-limit-period are experimental.
> +# @unstable: Members @x-checkpoint-delay,
> +#     @x-vcpu-dirty-limit-period and @backend-transfer are experimental.

List members in alphabetical order, please.

>  #
>  # Since: 2.4
>  ##
> @@ -950,7 +966,8 @@
>             'vcpu-dirty-limit',
>             'mode',
>             'zero-page-detection',
> -           'direct-io'] }
> +           'direct-io',
> +           'backend-transfer' ] }

Forgot feature 'unstable'?

>  
>  ##
>  # @MigrateSetParameters:
> @@ -1105,10 +1122,14 @@
>  #     only has effect if the @mapped-ram capability is enabled.
>  #     (Since 9.1)
>  #
> +# @backend-transfer: List of targets to enable backend-transfer
> +#     migration for. This requires migration channel to be a unix
> +#     socket (to pass fds through). (Since 10.2)
> +#
>  # Features:
>  #
> -# @unstable: Members @x-checkpoint-delay and
> -#     @x-vcpu-dirty-limit-period are experimental.
> +# @unstable: Members @x-checkpoint-delay,
> +#     @x-vcpu-dirty-limit-period and @backend-transfer are experimental.
>  #
>  # TODO: either fuse back into `MigrationParameters`, or make
>  #     `MigrationParameters` members mandatory
> @@ -1146,7 +1167,9 @@
>              '*vcpu-dirty-limit': 'uint64',
>              '*mode': 'MigMode',
>              '*zero-page-detection': 'ZeroPageDetection',
> -            '*direct-io': 'bool' } }
> +            '*direct-io': 'bool',
> +            '*backend-transfer': { 'type': [ 'BackendTransfer' ],
> +                                   'features': [ 'unstable' ] } } }
>  
>  ##
>  # @migrate-set-parameters:
> @@ -1315,10 +1338,14 @@
>  #     only has effect if the @mapped-ram capability is enabled.
>  #     (Since 9.1)
>  #
> +# @backend-transfer: List of targets to enable backend-transfer
> +#     migration for. This requires migration channel to be a unix
> +#     socket (to pass fds through). (Since 10.2)
> +#
>  # Features:
>  #
> -# @unstable: Members @x-checkpoint-delay and
> -#     @x-vcpu-dirty-limit-period are experimental.
> +# @unstable: Members @x-checkpoint-delay,
> +#     @x-vcpu-dirty-limit-period and @backend-transfer are experimental.
>  #
>  # Since: 2.4
>  ##
> @@ -1353,7 +1380,9 @@
>              '*vcpu-dirty-limit': 'uint64',
>              '*mode': 'MigMode',
>              '*zero-page-detection': 'ZeroPageDetection',
> -            '*direct-io': 'bool' } }
> +            '*direct-io': 'bool',
> +            '*backend-transfer': { 'type': [ 'BackendTransfer' ],
> +                                   'features': [ 'unstable' ] } } }
>  
>  ##
>  # @query-migrate-parameters: