Hi all!

Here is a new migration parameter backend-transfer, which, being
assisted by new device property backend-transfer, allows to
enable local migration of TAP virtio-net backend, including its
properties and open fds.

With this new option, management software doesn't need to
initialize new TAP and do a switch to it. Nothing should be
done around virtio-net in local migration: it just migrates
and continues to use same TAP device. So we avoid extra logic
in management software, extra allocations in kernel (for new TAP),
and corresponding extra delay in migration downtime.

v8:
14: add a-b by Peter
16: rework to one boolean parameter
17: rework to use per-device property
19: update to use new API

Vladimir Sementsov-Ogievskiy (19):
  net/tap: net_init_tap_one(): drop extra error propagation
  net/tap: net_init_tap_one(): move parameter checking earlier
  net/tap: rework net_tap_init()
  net/tap: pass NULL to net_init_tap_one() in cases when scripts are
    NULL
  net/tap: rework scripts handling
  net/tap: setup exit notifier only when needed
  net/tap: split net_tap_fd_init()
  net/tap: tap_set_sndbuf(): add return value
  net/tap: rework tap_set_sndbuf()
  net/tap: rework sndbuf handling
  net/tap: introduce net_tap_setup()
  net/tap: move vhost fd initialization to net_tap_new()
  net/tap: finalize net_tap_set_fd() logic
  migration: introduce .pre_incoming() vmsd handler
  net/tap: postpone tap setup to pre-incoming
  qapi: introduce backend-transfer migration parameter
  virtio-net: support backend-transfer migration for virtio-net/tap
  tests/functional: add skipWithoutSudo() decorator
  tests/functional: add test_x86_64_tap_migration

 hw/core/machine.c                             |   1 +
 hw/net/virtio-net.c                           | 151 ++++++-
 include/hw/virtio/virtio-net.h                |   1 +
 include/migration/vmstate.h                   |   1 +
 include/net/tap.h                             |   5 +
 migration/migration.c                         |   4 +
 migration/options.c                           |  18 +
 migration/options.h                           |   2 +
 migration/savevm.c                            |  15 +
 migration/savevm.h                            |   1 +
 net/tap-bsd.c                                 |   3 +-
 net/tap-linux.c                               |  19 +-
 net/tap-solaris.c                             |   3 +-
 net/tap-stub.c                                |   3 +-
 net/tap-win32.c                               |  11 +
 net/tap.c                                     | 425 +++++++++++++-----
 net/tap_int.h                                 |   3 +-
 qapi/migration.json                           |  38 +-
 tests/functional/qemu_test/decorators.py      |  16 +
 tests/functional/test_x86_64_tap_migration.py | 395 ++++++++++++++++
 20 files changed, 984 insertions(+), 131 deletions(-)
 create mode 100644 tests/functional/test_x86_64_tap_migration.py

-- 
2.48.1

Tested this series of patches with virtio-net regression tests,
everything works fine.

Tested-by: Lei Yang <leiyang@redhat.com>

On Wed, Oct 15, 2025 at 9:21 PM Vladimir Sementsov-Ogievskiy
<vsementsov@yandex-team.ru> wrote:
>
> Hi all!
>
> Here is a new migration parameter backend-transfer, which, being
> assisted by new device property backend-transfer, allows to
> enable local migration of TAP virtio-net backend, including its
> properties and open fds.
>
> With this new option, management software doesn't need to
> initialize new TAP and do a switch to it. Nothing should be
> done around virtio-net in local migration: it just migrates
> and continues to use same TAP device. So we avoid extra logic
> in management software, extra allocations in kernel (for new TAP),
> and corresponding extra delay in migration downtime.
>
> v8:
> 14: add a-b by Peter
> 16: rework to one boolean parameter
> 17: rework to use per-device property
> 19: update to use new API
>
> Vladimir Sementsov-Ogievskiy (19):
>   net/tap: net_init_tap_one(): drop extra error propagation
>   net/tap: net_init_tap_one(): move parameter checking earlier
>   net/tap: rework net_tap_init()
>   net/tap: pass NULL to net_init_tap_one() in cases when scripts are
>     NULL
>   net/tap: rework scripts handling
>   net/tap: setup exit notifier only when needed
>   net/tap: split net_tap_fd_init()
>   net/tap: tap_set_sndbuf(): add return value
>   net/tap: rework tap_set_sndbuf()
>   net/tap: rework sndbuf handling
>   net/tap: introduce net_tap_setup()
>   net/tap: move vhost fd initialization to net_tap_new()
>   net/tap: finalize net_tap_set_fd() logic
>   migration: introduce .pre_incoming() vmsd handler
>   net/tap: postpone tap setup to pre-incoming
>   qapi: introduce backend-transfer migration parameter
>   virtio-net: support backend-transfer migration for virtio-net/tap
>   tests/functional: add skipWithoutSudo() decorator
>   tests/functional: add test_x86_64_tap_migration
>
>  hw/core/machine.c                             |   1 +
>  hw/net/virtio-net.c                           | 151 ++++++-
>  include/hw/virtio/virtio-net.h                |   1 +
>  include/migration/vmstate.h                   |   1 +
>  include/net/tap.h                             |   5 +
>  migration/migration.c                         |   4 +
>  migration/options.c                           |  18 +
>  migration/options.h                           |   2 +
>  migration/savevm.c                            |  15 +
>  migration/savevm.h                            |   1 +
>  net/tap-bsd.c                                 |   3 +-
>  net/tap-linux.c                               |  19 +-
>  net/tap-solaris.c                             |   3 +-
>  net/tap-stub.c                                |   3 +-
>  net/tap-win32.c                               |  11 +
>  net/tap.c                                     | 425 +++++++++++++-----
>  net/tap_int.h                                 |   3 +-
>  qapi/migration.json                           |  38 +-
>  tests/functional/qemu_test/decorators.py      |  16 +
>  tests/functional/test_x86_64_tap_migration.py | 395 ++++++++++++++++
>  20 files changed, 984 insertions(+), 131 deletions(-)
>  create mode 100644 tests/functional/test_x86_64_tap_migration.py
>
> --
> 2.48.1
>