[dpdk-dev] [Bug 383] dpdk virtio_user lack of notifications make vhost_net+napi stops tx buffers

[bugzilla at dpdk.org]

https://bugs.dpdk.org/show_bug.cgi?id=383

            Bug ID: 383
           Summary: dpdk virtio_user lack of notifications make
                    vhost_net+napi stops tx buffers
           Product: DPDK
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: vhost/virtio
          Assignee: dev at dpdk.org
          Reporter: eupm90 at gmail.com
  Target Milestone: ---

Using the current testpmd vhost_user as:

./app/testpmd -l 6,7,8 --vdev='net_vhost1,iface=/tmp/vhost-user1'
--vdev='net_vhost2,iface=/tmp/vhost-user2' -- -a -i --rxq=1 --txq=1 --txd=1024
--forward-mode=rxonly

And starting qemu using packed=on on the interface:

-netdev vhost-user,chardev=charnet1,id=hostnet1 -device
virtio-net-pci,rx_queue_size=256,...,packed=on

And start to tx in the guest using:

./dpdk/build/app/testpmd -l 1,2 --vdev=eth_af_packet0,iface=eth0 -- \
    --forward-mode=txonly --txq=1 --txd=256 --auto-start --txpkts 1500 \
    --stats-period 1

After first burst of packets (512 or a little more), sendto() will start to
return EBUSY. kernel NAPI is refusing to send more packets to virtio_net device
until it free old skbs.

However, virtio_net driver is unable to free old buffers since host
does not return them in `vhost_flush_dequeue_packed` until shadow queue is full
except for MAX_PKT_BURST (32) packets.

Sometimes we are lucky and reach this point, or packets are small enough to
fill the queue and flush, but if the packets and the virtqueue are big enough,
we will not be able to tx anymore.

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