[PATCH] vhost: add VDUSE virtqueue ready state polling workaround

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Add workaround to poll virtqueue ready states before starting device
when VIRTIO_DEVICE_STATUS_DRIVER_OK is set in vduse_events_handler().

For each virtqueue, poll using VDUSE_VQ_GET_INFO ioctl to check
vq_info->ready state with configurable retry limit. This addresses
timing issues where device start was attempted before all virtqueues
were properly initialized and ready.

A notification mechanism will be introduced in the next version of
the VDUSE uAPI. When it lands, we would only apply this workaround
when the kernel does not support it.

Fixes: a9120db8b98b ("vhost: add VDUSE device startup")
Cc: stable@dpdk.org

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/vhost/vduse.c | 62 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 2 deletions(-)

diff --git a/lib/vhost/vduse.c b/lib/vhost/vduse.c
index 9de7f04a4f..5a6025d702 100644
--- a/lib/vhost/vduse.c
+++ b/lib/vhost/vduse.c
@@ -272,6 +272,56 @@ vduse_vring_cleanup(struct virtio_net *dev, unsigned int index)
 	vq->last_avail_idx = 0;
 }
 
+
+/*
+ * Tests show that it succeeds at the first retry at worst,
+ * but let's be on the safe side and allow more retries.
+ */
+#define VDUSE_VQ_READY_POLL_MAX_RETRIES 100
+
+static int
+vduse_wait_for_virtqueues_ready(struct virtio_net *dev)
+{
+	struct vduse_vq_info vq_info;
+	unsigned int i;
+	int ret;
+
+	for (i = 0; i < dev->nr_vring; i++) {
+		int retry_count = 0;
+
+		while (retry_count < VDUSE_VQ_READY_POLL_MAX_RETRIES) {
+			vq_info.index = i;
+			ret = ioctl(dev->vduse_dev_fd, VDUSE_VQ_GET_INFO, &vq_info);
+			if (ret) {
+				VHOST_CONFIG_LOG(dev->ifname, ERR,
+					"Failed to get VQ %u info while polling ready state: %s",
+					i, strerror(errno));
+				return -1;
+			}
+
+			if (vq_info.ready) {
+				VHOST_CONFIG_LOG(dev->ifname, DEBUG,
+					"VQ %u is ready after %u retries", i, retry_count);
+				break;
+			}
+
+			retry_count++;
+			/* Small delay between retries */
+			usleep(1000);
+		}
+
+		if (retry_count >= VDUSE_VQ_READY_POLL_MAX_RETRIES) {
+			VHOST_CONFIG_LOG(dev->ifname, ERR,
+				"VQ %u ready state polling timeout after %u retries",
+				i, VDUSE_VQ_READY_POLL_MAX_RETRIES);
+			return -1;
+		}
+	}
+
+	VHOST_CONFIG_LOG(dev->ifname, INFO, "All virtqueues are ready after polling");
+	return 0;
+}
+
 static void
 vduse_device_start(struct virtio_net *dev, bool reconnect)
 {
@@ -414,10 +464,18 @@ vduse_events_handler(int fd, void *arg, int *close __rte_unused)
 	}
 
 	if ((old_status ^ dev->status) & VIRTIO_DEVICE_STATUS_DRIVER_OK) {
-		if (dev->status & VIRTIO_DEVICE_STATUS_DRIVER_OK)
+		if (dev->status & VIRTIO_DEVICE_STATUS_DRIVER_OK) {
+			/* Poll virtqueues ready states before starting device */
+			ret = vduse_wait_for_virtqueues_ready(dev);
+			if (ret < 0) {
+				VHOST_CONFIG_LOG(dev->ifname, ERR,
+					"Failed to wait for virtqueues ready, aborting device start");
+				return;
+			}
 			vduse_device_start(dev, false);
-		else
+		} else {
 			vduse_device_stop(dev);
+		}
 	}
 
 	VHOST_CONFIG_LOG(dev->ifname, INFO, "Request %s (%u) handled successfully",
-- 
2.51.0