[PATCH RFC net-next] virtio_net: Relax queue requirement for using XDP

[David Ahern <dsahern@kernel.org>]

From: David Ahern <dahern@digitalocean.com>

virtio_net currently requires extra queues to install an XDP program,
with the rule being twice as many queues as vcpus. From a host
perspective this means the VM needs to have 2*vcpus vhost threads
for each guest NIC for which XDP is to be allowed. For example, a
16 vcpu VM with 2 tap devices needs 64 vhost threads.

The extra queues are only needed in case an XDP program wants to
return XDP_TX. XDP_PASS, XDP_DROP and XDP_REDIRECT do not need
additional queues. Relax the queue requirement and allow XDP
functionality based on resources. If an XDP program is loaded and
there are insufficient queues, then return a warning to the user
and if a program returns XDP_TX just drop the packet. This allows
the use of the rest of the XDP functionality to work without
putting an unreasonable burden on the host.

Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Ahern <dahern@digitalocean.com>
---
 drivers/net/virtio_net.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 2fe7a3188282..2f4c5b2e674d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -190,6 +190,8 @@ struct virtnet_info {
 	/* # of XDP queue pairs currently used by the driver */
 	u16 xdp_queue_pairs;
 
+	bool can_do_xdp_tx;
+
 	/* I like... big packets and I cannot lie! */
 	bool big_packets;
 
@@ -697,6 +699,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
 			len = xdp.data_end - xdp.data;
 			break;
 		case XDP_TX:
+			if (!vi->can_do_xdp_tx)
+				goto err_xdp;
 			stats->xdp_tx++;
 			xdpf = convert_to_xdp_frame(&xdp);
 			if (unlikely(!xdpf))
@@ -870,6 +874,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			}
 			break;
 		case XDP_TX:
+			if (!vi->can_do_xdp_tx)
+				goto err_xdp;
 			stats->xdp_tx++;
 			xdpf = convert_to_xdp_frame(&xdp);
 			if (unlikely(!xdpf))
@@ -2435,10 +2441,10 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 
 	/* XDP requires extra queues for XDP_TX */
 	if (curr_qp + xdp_qp > vi->max_queue_pairs) {
-		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available");
-		netdev_warn(dev, "request %i queues but max is %i\n",
-			    curr_qp + xdp_qp, vi->max_queue_pairs);
-		return -ENOMEM;
+		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available; XDP_TX will not be allowed");
+		vi->can_do_xdp_tx = false;
+	} else {
+		vi->can_do_xdp_tx = true;
 	}
 
 	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
-- 
2.17.1