[PATCH 2/2] net/tap: drain queue FD in Rx interrupt mode

[Maxime Leroy]

The tap Rx path is driven by a SIGIO trigger: pmd_rx_burst() returns
without reading the queue fd unless tap_trigger, bumped by the O_ASYNC
signal handler, has advanced. That avoids a readv() on every empty poll.

In Rx interrupt mode the application blocks on the queue fd through epoll
and polls only after a wakeup. The epoll wakeup and the trigger are
distinct signals, so the burst can return 0 right after a wakeup because
the trigger has not advanced, leaving the fd readable with no further
edge: traffic stalls.

When the port is configured for Rx interrupts, drain the fd
unconditionally in the burst and do not arm the SIGIO trigger on the data
queue fd. A data queue fd can be closed and recreated on a later setup,
so the mode must stay constant to keep every fd on the same SIGIO policy:
it is fixed at configure time and a later change is rejected. Close and
reopen the port to switch modes.

Fixes: 4870a8cdd968 ("net/tap: support Rx interrupt")
Cc: stable at dpdk.org

Signed-off-by: Maxime Leroy <maxime at leroys.fr>
---
 drivers/net/tap/rte_eth_tap.c | 18 +++++++++++++++++-
 drivers/net/tap/rte_eth_tap.h |  2 ++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
index 99ede19e49..ea9ef76335 100644
--- a/drivers/net/tap/rte_eth_tap.c
+++ b/drivers/net/tap/rte_eth_tap.c
@@ -254,6 +254,10 @@ tun_alloc(struct pmd_internals *pmd, int is_keepalive, int persistent)
 		goto error;
 	}
 
+	/* interrupt mode wakes through epoll on the data queue fd, not the SIGIO trigger */
+	if (pmd->intr_mode)
+		return fd;
+
 	/* Find a free realtime signal */
 	for (signo = SIGRTMIN + 1; signo < SIGRTMAX; signo++) {
 		struct sigaction sa;
@@ -477,7 +481,7 @@ pmd_rx_burst(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 	unsigned long num_rx_bytes = 0;
 	uint32_t trigger = tap_trigger;
 
-	if (trigger == rxq->trigger_seen)
+	if (!rxq->intr_mode && trigger == rxq->trigger_seen)
 		return 0;
 
 	process_private = rte_eth_devices[rxq->in_port].process_private;
@@ -937,6 +941,16 @@ tap_dev_configure(struct rte_eth_dev *dev)
 	struct pmd_internals *pmd = dev->data->dev_private;
 	int intr_mode = !!dev->data->dev_conf.intr_conf.rxq;
 
+	/* The queue fd is created once and its SIGIO trigger is armed for poll
+	 * mode only; the interrupt mode cannot be toggled on an existing port.
+	 */
+	if (pmd->intr_mode_set && pmd->intr_mode != intr_mode) {
+		TAP_LOG(ERR,
+			"%s: Rx interrupt mode is fixed after configure, close and reopen the port to change it",
+			dev->device->name);
+		return -ENOTSUP;
+	}
+
 	if (dev->data->nb_rx_queues != dev->data->nb_tx_queues) {
 		TAP_LOG(ERR,
 			"%s: number of rx queues %d must be equal to number of tx queues %d",
@@ -947,6 +961,7 @@ tap_dev_configure(struct rte_eth_dev *dev)
 	}
 
 	pmd->intr_mode = intr_mode;
+	pmd->intr_mode_set = 1;
 
 	TAP_LOG(INFO, "%s: %s: TX configured queues number: %u",
 		dev->device->name, pmd->name, dev->data->nb_tx_queues);
@@ -1620,6 +1635,7 @@ tap_rx_queue_setup(struct rte_eth_dev *dev,
 	rxq->queue_id = rx_queue_id;
 	rxq->max_rx_segs = max_rx_segs;
 	rxq->rxmode = &dev->data->dev_conf.rxmode;
+	rxq->intr_mode = internals->intr_mode;
 
 	dev->data->rx_queues[rx_queue_id] = rxq;
 	int fd = tap_setup_queue(dev, rx_queue_id, 1);
diff --git a/drivers/net/tap/rte_eth_tap.h b/drivers/net/tap/rte_eth_tap.h
index 3180719c34..abe22aac9f 100644
--- a/drivers/net/tap/rte_eth_tap.h
+++ b/drivers/net/tap/rte_eth_tap.h
@@ -50,6 +50,7 @@ struct rx_queue {
 	uint16_t queue_id;		/* queue ID*/
 	struct queue_stats stats;        /* Stats for this RX queue */
 	uint16_t max_rx_segs;           /* max scatter segments per packet */
+	uint16_t intr_mode;             /* 1 when Rx queue interrupts are used */
 	struct rte_eth_rxmode *rxmode;  /* RX features */
 	struct rte_mbuf *pool;          /* mbufs pool for this queue */
 	struct tun_pi pi;               /* packet info for iovecs */
@@ -94,6 +95,7 @@ struct pmd_internals {
 
 	struct rte_intr_handle *intr_handle;         /* LSC interrupt handle. */
 	int intr_mode;                    /* Rx queue interrupt mode */
+	int intr_mode_set;                /* intr_mode locked after configure */
 	int ka_fd;                        /* keep-alive file descriptor */
 	struct rte_mempool *gso_ctx_mp;     /* Mempool for GSO packets */
 };
-- 
2.43.0