xdp: add XDP mode detection via knot_eth_xdp_mode()

distro/deb/libknot11.symbols

@@ -80,6 +80,7 @@ libknot.so.11 libknot11 #MINVER#
  knot_error_from_libdnssec@Base 3.0.0
  knot_eth_name_from_addr@Base 3.0.0
  knot_eth_queues@Base 3.0.0
+ knot_eth_xdp_mode@Base 3.0.2
  knot_get_obsolete_rdata_descriptor@Base 3.0.0
  knot_get_rdata_descriptor@Base 3.0.0
  knot_naptr_header_size@Base 3.0.0

src/libknot/xdp/eth.c

@@ -14,10 +14,12 @@
     along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+#include <bpf/libbpf.h>
 #include <errno.h>
 #include <ifaddrs.h>
 #include <linux/ethtool.h>
 #include <linux/if.h>
+#include <linux/if_link.h>
 #include <linux/sockios.h>
 #include <sys/ioctl.h>
 #include <unistd.h>
@@ -105,3 +107,23 @@ int knot_eth_name_from_addr(const struct sockaddr_storage *addr, char *out,
 	freeifaddrs(ifaces);
 	return matches == 0 ? KNOT_EADDRNOTAVAIL : KNOT_ELIMIT;
 }
+
+_public_
+knot_xdp_mode_t knot_eth_xdp_mode(int if_index)
+{
+	struct xdp_link_info info;
+	int ret = bpf_get_link_xdp_info(if_index, &info, sizeof(info), 0);
+	if (ret != 0) {
+		return KNOT_XDP_MODE_NONE;
+	}
+
+	switch (info.attach_mode) {
+	case XDP_ATTACHED_DRV:
+	case XDP_ATTACHED_HW:
+		return KNOT_XDP_MODE_FULL;
+	case XDP_ATTACHED_SKB:
+		return KNOT_XDP_MODE_EMUL;
+	default:
+		return KNOT_XDP_MODE_NONE;
+	}
+}

src/libknot/xdp/eth.h

@@ -40,3 +40,18 @@ int knot_eth_queues(const char *devname);
  */
 int knot_eth_name_from_addr(const struct sockaddr_storage *addr, char *out,
                             size_t out_len);
+
+typedef enum {
+	KNOT_XDP_MODE_NONE, /*!< XDP not available or error. */
+	KNOT_XDP_MODE_FULL, /*!< Full XDP support in driver or HW. */
+	KNOT_XDP_MODE_EMUL, /*!< Emulated XDP support. */
+} knot_xdp_mode_t;
+
+/*!
+ * \brief Check if the network inteface supports XDP.
+ *
+ * \param if_index  Index of interface, output from if_nametoindex().
+ *
+ * \return Supported XDP mode.
+ */
+knot_xdp_mode_t knot_eth_xdp_mode(int if_index);