Reconnect failed with loopback virtio1.1 server mode test

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1863200
Reported by 'xuan' on 2020-02-14 :

Issue discription：
Packed ring server mode is a new feature to enable the virtio-user or
virtio-pmd(in VM) as the server, vhost as the client, then when the vhost-
user is killed then re-launched, the vhost-user can connect back to
virtio-user/virtio-pmd again. Test with dpdk 20.02 ,virtio-pmd loopback
reconnect from vhost-user failed.

Test Environment:
DPDK version: DPDK v20.02
Other software versions: virtio1.1
Qemu versions:4.2.0
OS: Linux 4.15.0-20-generic
Compiler: gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
Hardware platform: R2208WFTZSR.

The reproduce step is :
Test Case: vhost-user/virtio-pmd loopback reconnect from vhost-user
===============================================================
Flow: Vhost-user --> Virtio --> Vhost-user

1. Launch vhost-user with client mode by below commands::

    ./testpmd -c 0x30 -n 4 --socket-mem 1024,1024 --legacy-mem --vdev
'eth_vhost0,iface=/tmp/vhost-net,client=1,queues=1' -- -i --nb-cores=1
    testpmd>set fwd mac

2. Start VM with 1 virtio device, and set the qemu as server mode::

    ./qemu-system-x86_64 -name vm2 -enable-kvm -cpu host -smp 100 -m 8G \
    -object memory-backend-file,id=mem,size=8192M,mem-
path=/mnt/huge,share=on \
    -numa node,memdev=mem -mem-prealloc -drive
file=/home/xuan/dpdk_project/shell/u18.img  \
    -chardev socket,path=/tmp/vm2_qga0.sock,server,nowait,id=vm2_qga0
-device virtio-serial \
    -device virtserialport,chardev=vm2_qga0,name=org.qemu.guest_agent.2
-daemonize \
    -monitor unix:/tmp/vm2_monitor.sock,server,nowait -net
nic,macaddr=00:00:00:08:e8:aa,addr=1f \
    -net user,hostfwd=tcp:127.0.0.1:6002-:22 \
    -chardev socket,id=char0,path=/tmp/vhost-net,server \
    -netdev type=vhost-user,id=netdev0,chardev=char0,vhostforce \
    -device virtio-net-
pci,netdev=netdev0,mac=52:54:00:00:00:01,mrg_rxbuf=on,rx_queue_size=1024,tx_queue_size=1024,packed=on
\
    -vnc :10

3. On VM, bind virtio net to igb_uio and run testpmd::

    ./testpmd -c 0x3 -n 4 -- -i --nb-cores=1 --txd=1024 --rxd=1024
    testpmd>set fwd mac
    testpmd>start

4. Send packets by vhost-user, check if packets can be RX/TX with virtio-
pmd::

    testpmd>start tx_first 32
    testpmd>show port stats all

5. On host, quit vhost-user, then re-launch the vhost-user with below
command::

    testpmd>quit
    ./testpmd -c 0x30 -n 4 --socket-mem 1024,1024 --legacy-mem --vdev
'eth_vhost0,iface=/tmp/vhost-net,client=1,queues=1' -- -i --nb-cores=1
    testpmd>set fwd mac
    testpmd>start tx_first 32

6. Check if the reconnection can work, still send packets by vhost-user,
check if packets can be RX/TX with virtio-pmd::

    testpmd>show port stats all

Expected result::

After the vhost-user is killed then re-launched, the VM can connect back
to vhost-user again with throughput.

Real result::

After the vhost-user is killed then re-launched, no throughput with PVP.

Analysis::

QEMU has its own way to handle reconnect function for virtio server mode.
However, for packed ring, when reconnecting to virtio, vhost cannot get
the status of descriptors via the descriptor ring. This bug is caused
since the reconnection for packed ring need additional reset operation.