eth: fbnic: Add XDP pass, drop, abort support

eth: fbnic: Add XDP pass, drop, abort support

Add basic support for attaching an XDP program to the device and support
for PASS/DROP/ABORT actions. In fbnic, buffers are always mapped as
DMA_BIDIRECTIONAL.

The BPF program pointer can be read either on a per-packet basis or on a
per-NAPI poll basis. Both approaches are functionally equivalent, in the
current code. Stick to per-packet as it limits number of arguments we need
to pass around.

On the XDP hot path, check that packets with fragments are only allowed
when multi-buffer support is enabled for the XDP program. Ideally, this
check should not be necessary because ndo_bpf verifies that for XDP
programs without multi-buff support, MTU is less than the hds_thresh.
However, the MTU currently does not enforce the receive size which would
require cleaning up the data path and bouncing the link. For practical
reasons, prioritize the ability to enter and exit BPF mode with different
MTU sizes without requiring a full reconfig.

Testing:

Hook a simple XDP program that passes all the packets destined for a
specific port

iperf3 -c 192.168.1.10 -P 5 -p 12345
Connecting to host 192.168.1.10, port 12345
[  5] local 192.168.1.9 port 46702 connected to 192.168.1.10 port 12345
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
- - - - - - - - - - - - - - - - - - - - - - - - -
[SUM]   1.00-2.00   sec  3.86 GBytes  33.2 Gbits/sec    0

XDP_DROP:
Hook an XDP program that drops packets destined for a specific port

 iperf3 -c 192.168.1.10 -P 5 -p 12345
^C- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[SUM]   0.00-0.00   sec  0.00 Bytes  0.00 bits/sec    0       sender
[SUM]   0.00-0.00   sec  0.00 Bytes  0.00 bits/sec            receiver
iperf3: interrupt - the client has terminated

XDP with HDS:

- Validate XDP attachment failure when HDS is low
   ~] ethtool -G eth0 hds-thresh 512
   ~] sudo ip link set eth0 xdpdrv obj xdp_pass_12345.o sec xdp
   ~] Error: fbnic: MTU too high, or HDS threshold is too low for single
      buffer XDP.

- Validate successful XDP attachment when HDS threshold is appropriate
  ~] ethtool -G eth0 hds-thresh 1536
  ~] sudo ip link set eth0 xdpdrv obj xdp_pass_12345.o sec xdp

- Validate when the XDP program is attached, changing HDS thresh to a
  lower value fails
  ~] ethtool -G eth0 hds-thresh 512
  ~] netlink error: fbnic: Use higher HDS threshold or multi-buf capable
     program

- Validate HDS thresh does not matter when xdp frags support is
  available
  ~] ethtool -G eth0 hds-thresh 512
  ~] sudo ip link set eth0 xdpdrv obj xdp_pass_mb_12345.o sec xdp.frags

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20250813221319.3367670-6-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>