net: implement virtio helpers to handle UDP GSO tunneling.

net: implement virtio helpers to handle UDP GSO tunneling.

The virtio specification are introducing support for GSO over UDP
tunnel.

This patch brings in the needed defines and the additional virtio hdr
parsing/building helpers.

The UDP tunnel support uses additional fields in the virtio hdr, and such
fields location can change depending on other negotiated features -
specifically VIRTIO_NET_F_HASH_REPORT.

Try to be as conservative as possible with the new field validation.

Existing implementation for plain GSO offloads allow for invalid/
self-contradictory values of such fields. With GSO over UDP tunnel we can
be more strict, with no need to deal with legacy implementation.

Since the checksum-related field validation is asymmetric in the driver
and in the device, introduce a separate helper to implement the new checks
(to be used only on the driver side).

Note that while the feature space exceeds the 64-bit boundaries, the
guest offload space is fixed by the specification of the
VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET command to a 64-bit size.

Prior to the UDP tunnel GSO support, each guest offload bit corresponded
to the feature bit with the same value and vice versa.

Due to the limited 'guest offload' space, relevant features in the high
64 bits are 'mapped' to free bits in the lower range. That is simpler
than defining a new command (and associated features) to exchange an
extended guest offloads set.

As a consequence, the uAPIs also specify the mapped guest offload value
corresponding to the UDP tunnel GSO features.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
--
v4 -> v5:
  - avoid lines above 80 chars

v3 -> v4:
  - fixed offset for UDP GSO tunnel, update accordingly the helpers
  - tried to clarified vlan_hlen semantic
  - virtio_net_chk_data_valid() -> virtio_net_handle_csum_offload()

v2 -> v3:
  - add definitions for possible vnet hdr layouts with tunnel support

v1 -> v2:
  - 'relay' -> 'rely' typo
  - less unclear comment WRT enforced inner GSO checks
  - inner header fields are allowed only with 'modern' virtio,
    thus are always le
  - clarified in the commit message the need for 'mapped features'
    defines
  - assume little_endian is true when UDP GSO is enabled.
  - fix inner proto type value