netdev-afxdp: Add need_wakeup support.

The patch adds support for using need_wakeup flag in AF_XDP rings.
A new option, use-need-wakeup, is added.  When this option is used,
it means that OVS has to explicitly wake up the kernel RX, using poll()
syscall and wake up TX, using sendto() syscall. This feature improves
the performance by avoiding unnecessary sendto syscalls for TX.
For RX, instead of kernel always busy-spinning on fille queue, OVS wakes
up the kernel RX processing when fill queue is replenished.

The need_wakeup feature is merged into Linux kernel bpf-next tee with commit
77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings") and
OVS enables it by default, if libbpf supports it.  If users enable it but
runs in an older version of libbpf, then the need_wakeup feature has no effect,
and a warning message is logged.

For virtual interface, it's better set use-need-wakeup=false, since
the virtual device's AF_XDP xmit is synchronous: the sendto syscall
enters kernel and process the TX packet on tx queue directly.

On Intel Xeon E5-2620 v3 2.4GHz system, performance of physical port
to physical port improves from 6.1Mpps to 7.3Mpps.

Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/netdev-linux-private.h

@@ -102,6 +102,8 @@ struct netdev_linux {
     int requested_n_rxq;
     int xdpmode;                /* AF_XDP running mode: driver or skb. */
     int requested_xdpmode;
+    bool use_need_wakeup;
+    bool requested_need_wakeup;
     struct ovs_spin *tx_locks;  /* spin lock array for TX queues. */
 #endif
 };