On Mon, Oct 06, 2025 at 03:23:06PM +0200, Markus Armbruster wrote:
> Vladimir Sementsov-Ogievskiy <[email protected]> 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 <[email protected]>
> > ---
> > 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.
However, QAPI_LIST_FOR_EACH can potentially be used to implement
QAPI_LIST_LENGTH.
#define QAPI_LIST_FOR_EACH(list, tail) \
for (tail = list; tail != NULL; tail = tail->next)
and
#define QAPI_LIST_LENGTH(list) \
({ \
size_t _len = 0; \
typeof_strip_qual(list) _tail; \
QAPI_LIST_FOR_EACH(list, tail) { \
_len++; \
} \
_len; \
})
> 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:
