[PATCH v2 0/3] net/tap, virtio-net: auto-detect MTU from host tap device

[Nariman Sayed <narimansayed28@gmail.com>]

This series adds automatic detection of the host tap device's MTU
and exposes it to the guest via VIRTIO_NET_F_MTU, addressing
gitlab.com/qemu-project/qemu/-/issues/3155.

Currently, host_mtu= must be set manually on the virtio-net-pci
device, duplicating the MTU already configured on the host's tap
interface. This series adds a new mtu-from-tap boolean property
(default off) that, when enabled, automatically reads the backend's
MTU and uses it for the guest, avoiding the need to specify the
value twice.

v2:
 - Addressed feedback from Eugenio Perez Martin: rather than
   checking for NET_CLIENT_DRIVER_TAP and calling tap_get_mtu()
   directly, added a generic get_mtu callback to NetClientInfo and
   a qemu_get_mtu() wrapper, following the existing qemu_has_ufo()
   pattern. virtio-net.c now calls qemu_get_mtu() without any
   backend-type check, so other backends (e.g. vhost-user,
   vhost-vdpa) can implement get_mtu in the future without
   requiring any change to virtio-net.c. (patch 2 is new as a
   result; patch 3 updated accordingly)

The series is split into three patches:
 - patch 1 adds a helper to query a tap fd's host MTU (Linux only,
   with build-safe stubs for BSD/Solaris/stub backends)
 - patch 2 adds a generic get_mtu NetClientInfo callback and wires
   it up for the tap backend
 - patch 3 adds the new virtio-net property and wires it up,
   including a note on a config-space sizing timing issue
   discovered during testing

Tested manually with a tap device configured for MTU 9000 and an
Alpine Linux guest, confirming the guest correctly negotiates
VIRTIO_NET_F_MTU and reports the expected MTU. Also tested by
Aniket Sahu on ppc64le/POWER10 with a SLES guest.

Nariman Sayed (3):
  net/tap: add tap_fd_get_mtu() to query host tap MTU
  net: add generic get_mtu NetClientInfo callback
  virtio-net: add mtu-from-tap property for automatic MTU detection

 hw/net/virtio-net.c            | 10 ++++++++++
 include/hw/virtio/virtio-net.h |  1 +
 include/net/net.h              |  3 +++
 net/net.c                      |  9 +++++++++
 net/tap-bsd.c                  |  5 +++++
 net/tap-linux.c                | 30 ++++++++++++++++++++++++++++++
 net/tap-solaris.c              |  5 +++++
 net/tap-stub.c                 |  5 +++++
 net/tap.c                      |  8 ++++++++
 net/tap_int.h                  |  1 +
 10 files changed, 77 insertions(+)

-- 
2.34.1