selftests: drv-net: probe for AF_XDP sockets more explicitly

Separate the support check from socket binding for easier refactoring.
Use: ./helper - - just to probe if we can open the socket.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Tested-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250219234956.520599-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/queues.py

@@ -25,6 +25,13 @@ def nl_get_queues(cfg, nl, qtype='rx'):
     return None
 
 def check_xdp(cfg, nl, xdp_queue_id=0) -> None:
+    # Probe for support
+    xdp = cmd(cfg.rpath("xdp_helper") + ' - -', fail=False)
+    if xdp.ret == 255:
+        raise KsftSkipEx('AF_XDP unsupported')
+    elif xdp.ret > 0:
+        raise KsftFailEx('unable to create AF_XDP socket')
+
     xdp = subprocess.Popen([cfg.rpath("xdp_helper"), f"{cfg.ifindex}", f"{xdp_queue_id}"],
                            stdin=subprocess.PIPE, stdout=subprocess.PIPE, bufsize=1,
                            text=True)
@@ -33,11 +40,6 @@ def check_xdp(cfg, nl, xdp_queue_id=0) -> None:
     stdout, stderr = xdp.communicate(timeout=10)
     rx = tx = False
 
-    if xdp.returncode == 255:
-        raise KsftSkipEx('AF_XDP unsupported')
-    elif xdp.returncode > 0:
-        raise KsftFailEx('unable to create AF_XDP socket')
-
     queues = nl.queue_get({'ifindex': cfg.ifindex}, dump=True)
     if not queues:
         raise KsftSkipEx("Netlink reports no queues")