[PATCH] [PATCH net-next] tun: fix multiqueue rx

[Matthew Cover]

When writing packets to a descriptor associated with a combined queue, the
packets should end up on that queue.

Before this change all packets written to any descriptor associated with a
tap interface end up on rx-0, even when the descriptor is associated with a
different queue.

The rx traffic can be generated by either of the following.
  1. a simple tap program which spins up multiple queues and writes packets
     to each of the file descriptors
  2. tx from a qemu vm with a tap multiqueue netdev

The queue for rx traffic can be observed by either of the following (done
on the hypervisor in the qemu case).
  1. a simple netmap program which opens and reads from per-queue
     descriptors
  2. configuring RPS and doing per-cpu captures with rxtxcpu

Alternatively, if you printk() the return value of skb_get_rx_queue() just
before each instance of netif_receive_skb() in tun.c, you will get 65535
for every skb.

Calling skb_record_rx_queue() to set the rx queue to the queue_index fixes
the association between descriptor and rx queue.

Signed-off-by: Matthew Cover <matthew.co...@stackpath.com>
---
 drivers/net/tun.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index a65779c6d72f..ce8620f3ea5e 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1536,6 +1536,7 @@ static void tun_rx_batched(struct tun_struct *tun, struct 
tun_file *tfile,
 
        if (!rx_batched || (!more && skb_queue_empty(queue))) {
                local_bh_disable();
+               skb_record_rx_queue(skb, tfile->queue_index);
                netif_receive_skb(skb);
                local_bh_enable();
                return;
@@ -1555,8 +1556,11 @@ static void tun_rx_batched(struct tun_struct *tun, 
struct tun_file *tfile,
                struct sk_buff *nskb;
 
                local_bh_disable();
-               while ((nskb = __skb_dequeue(&process_queue)))
+               while ((nskb = __skb_dequeue(&process_queue))) {
+                       skb_record_rx_queue(nskb, tfile->queue_index);
                        netif_receive_skb(nskb);
+               }
+               skb_record_rx_queue(skb, tfile->queue_index);
                netif_receive_skb(skb);
                local_bh_enable();
        }
@@ -2452,6 +2456,7 @@ static int tun_xdp_one(struct tun_struct *tun,
            !tfile->detached)
                rxhash = __skb_get_hash_symmetric(skb);
 
+       skb_record_rx_queue(skb, tfile->queue_index);
        netif_receive_skb(skb);
 
        stats = get_cpu_ptr(tun->pcpu_stats);
-- 
2.15.2 (Apple Git-101.1)