RFC add partial rx hw csum offload support for XDP

Enable xdp rx checksum offload support for CHECKSUM_UNNECESSARY use-case. Introduce flag field in xdp_buff/xdp_frame in order to save the checksum result from the NIC and have a fast access to it performing XDP_REDIRECT. CHECKSUM_COMPLETE is not supported yet since it will require adding the csum result to the xdp_metadata area.
Moreover flag field will be reused for xdp multi-buff support. This series has been tested generating UDP traffic with pktgen and performing a xdp_redirect from an ixgbe device to a remote CPUMAP entry. PPS results show a negligible penalty respect to the baseline where the UDP checksum has been disabled. More info about the test can be found here [0].

[0] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp_frame01_checksum.org

Lorenzo Bianconi (4):
net: xdp: introduce flags field in xdp_buff and xdp_frame
mvneta: return csum computation result from mvneta_rx_csum
net: mvneta: report csum result in xdp_buff
net: xdp: update csum building the skb

 drivers/net/ethernet/marvell/mvneta.c | 27 ++++++++------------
 include/net/xdp.h                     | 36 +++++++++++++++++++++++++++
 net/core/xdp.c                        |  2 +-
 3 files changed, 48 insertions(+), 17 deletions(-)

2.31.1