[PATCH 0/2] fix AF_PACKET PMD rollover fanout mode

[PATCH 0/2] fix AF_PACKET PMD rollover fanout mode

[Frank Dressler]

This series fixes a bug in the AF_PACKET PMD that prevented it from
running in rollover fanout mode.

History: 364e08f2bb introduced the AF_PACKET PMD in 2014 and
unconditionally set the rollover flag. In 2025, d3bc77ab05 added
code allowing the user to select the fanout mode, including rollover
mode. Both the flag and the mode were introduced in Linux in 2013
with 77f65ebdca but their combination has been rejected ever since.
Because of that, rollover mode has never worked in the PMD.

The first patch clears the rollover _flag_ when the rollover _mode_
is set.

The second patch adds a test that checks that each fanout mode the
PMD supports is accepted by the kernel.

Frank Dressler (2):
  net/af_packet: do not set rollover flag in rollover mode
  test/af_packet: add test for fanout modes

 app/test/test_pmd_af_packet.c             | 41 +++++++++++++++++++++++
 drivers/net/af_packet/rte_eth_af_packet.c |  2 ++
 2 files changed, 43 insertions(+)

-- 
2.56.0

[PATCH 1/2] net/af_packet: do not set rollover flag in rollover mode

[Frank Dressler]

The kernel rejects fanout configurations in which both the rollover
mode and the rollover flag are set. Because the PMD always set the
flag, the mode never worked.

Fixes: d3bc77ab05a4 ("net/af_packet: allow changing fanout mode")
Cc: tudor.cornea@gmail.com
Cc: stable@dpdk.org

Signed-off-by: Frank Dressler <frank@dressler.pro>
---
 drivers/net/af_packet/rte_eth_af_packet.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
index ef57719803..870536a8b2 100644
--- a/drivers/net/af_packet/rte_eth_af_packet.c
+++ b/drivers/net/af_packet/rte_eth_af_packet.c
@@ -844,6 +844,8 @@ get_fanout_mode(const char *fanout_mode)
 	} else if (!strcmp(fanout_mode, "cpu")) {
 		load_balance |= PACKET_FANOUT_CPU;
 	} else if (!strcmp(fanout_mode, "rollover")) {
+		/* setting both is rejected by kernel */
+		load_balance &= ~PACKET_FANOUT_FLAG_ROLLOVER;
 		load_balance |= PACKET_FANOUT_ROLLOVER;
 	} else if (!strcmp(fanout_mode, "rnd")) {
 		load_balance |= PACKET_FANOUT_RND;
-- 
2.56.0

[PATCH 2/2] test/af_packet: add test for fanout modes

[Frank Dressler]

The added test checks if the kernel accepts each of the fanout
modes the AF_PACKET PMD supports.

Signed-off-by: Frank Dressler <frank@dressler.pro>
---
 app/test/test_pmd_af_packet.c | 41 +++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/app/test/test_pmd_af_packet.c b/app/test/test_pmd_af_packet.c
index b668ca5b81..478a0a6355 100644
--- a/app/test/test_pmd_af_packet.c
+++ b/app/test/test_pmd_af_packet.c
@@ -992,6 +992,46 @@ test_af_packet_multi_queue(void)
 	return TEST_SUCCESS;
 }
 
+/*
+ * Test: Fanout modes
+ */
+static int
+test_af_packet_fanout_modes(void)
+{
+	static const char * const modes[] = {
+		"hash", "lb", "cpu", "rollover", "rnd", "qm",
+	};
+	uint16_t test_port;
+	char devargs[256];
+	unsigned int i;
+	int ret;
+
+	if (!tap_created) {
+		printf("SKIPPED: TAP interface not available (need root)\n");
+		return TEST_SKIPPED;
+	}
+
+	for (i = 0; i < RTE_DIM(modes); i++) {
+		snprintf(devargs, sizeof(devargs), "iface=%s,qpairs=2,fanout_mode=%s",
+			 TAP_DEV_NAME, modes[i]);
+
+		ret = rte_vdev_init("net_af_packet_fanout", devargs);
+		TEST_ASSERT(ret == 0,
+			    "Failed to create af_packet sockets with fanout_mode=%s: %d",
+			    modes[i], ret);
+
+		ret = rte_eth_dev_get_port_by_name("net_af_packet_fanout", &test_port);
+		if (ret == 0)
+			rte_eth_dev_close(test_port);
+		rte_vdev_uninit("net_af_packet_fanout");
+		TEST_ASSERT(ret == 0,
+			    "af_packet port with fanout_mode=%s has no port id: %d",
+			    modes[i], ret);
+	}
+
+	return TEST_SUCCESS;
+}
+
 /*
  * Multi-threaded test context
  */
@@ -1108,6 +1148,7 @@ static struct unit_test_suite af_packet_test_suite = {
 		TEST_CASE(test_af_packet_frame_config),
 		TEST_CASE(test_af_packet_qdisc_bypass),
 		TEST_CASE(test_af_packet_multi_queue),
+		TEST_CASE(test_af_packet_fanout_modes),
 
 		TEST_CASES_END() /**< NULL terminate unit test array */
 	}
-- 
2.56.0

Re: [PATCH 0/2] fix AF_PACKET PMD rollover fanout mode

[Stephen Hemminger]

On Fri, 23 Oct 2026 17:22:45 +0100
Frank Dressler <frank@dressler.pro> wrote:

> This series fixes a bug in the AF_PACKET PMD that prevented it from
> running in rollover fanout mode.
> 
> History: 364e08f2bb introduced the AF_PACKET PMD in 2014 and
> unconditionally set the rollover flag. In 2025, d3bc77ab05 added
> code allowing the user to select the fanout mode, including rollover
> mode. Both the flag and the mode were introduced in Linux in 2013
> with 77f65ebdca but their combination has been rejected ever since.
> Because of that, rollover mode has never worked in the PMD.
> 
> The first patch clears the rollover _flag_ when the rollover _mode_
> is set.
> 
> The second patch adds a test that checks that each fanout mode the
> PMD supports is accepted by the kernel.
> 
> Frank Dressler (2):
>   net/af_packet: do not set rollover flag in rollover mode
>   test/af_packet: add test for fanout modes

LGTM - applied to next-net