[Bug 1888818] Re: Multi-queue vhost-user fails to reconnect with qemu version >=4.2

[xuan <1888818@bugs.launchpad.net>]

** Changed in: qemu
       Status: Incomplete => New

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https://bugs.launchpad.net/bugs/1888818

Title:
  Multi-queue vhost-user fails to reconnect with qemu version >=4.2

Status in QEMU:
  New

Bug description:
  Test Environment:
  DPDK version: DPDK v20.08
  Other software versions: qemu4.2.0, qemu5.0.0.
  OS: Linux 4.15.0-20-generic
  Compiler: gcc (Ubuntu 7.3.0-16ubuntu3) 8.4.0
  Hardware platform: Purley.
  Test Setup
  Steps to reproduce
  List the steps to reproduce the issue.

  Test flow
  =========
  1. Launch vhost-user testpmd as port0 with 2 queues:

  ./x86_64-native-linuxapp-gcc/app/testpmd -l 2-4 -n 4 \
      --file-prefix=vhost --vdev 'net_vhost0,iface=vhost-net,queues=2,client=1' -- -i --txd=1024 --rxd=1024 --txq=2 --rxq=2
  testpmd>start

  3. Launch qemu with virtio-net:

   taskset -c 13 \
      qemu-system-x86_64 -name us-vhost-vm1 \
         -cpu host -enable-kvm -m 2048 -object memory-backend-file,id=mem,size=2048M,mem-path=/mnt/huge,share=on \
         -numa node,memdev=mem \
         -mem-prealloc -monitor unix:/tmp/vm2_monitor.sock,server,nowait -netdev user,id=yinan,hostfwd=tcp:127.0.0.1:6005-:22 -device e1000,netdev=yinan \
         -smp cores=1,sockets=1 -drive file=/home/osimg/ubuntu16.img  \
         -chardev socket,id=char0,path=./vhost-net,server \
         -netdev type=vhost-user,id=mynet1,chardev=char0,vhostforce,queues=2 \
         -device virtio-net-pci,mac=52:54:00:00:00:01,netdev=mynet1,mrg_rxbuf=on,csum=on,gso=on,host_tso4=on,guest_tso4=on,mq=on,vectors=15 \
         -vnc :10 -daemonize

  6. Quit testpmd and restart vhost-user :

  testpmd>quit
  ./x86_64-native-linuxapp-gcc/app/testpmd -l 2-4 -n 4 \
      --file-prefix=vhost --vdev 'net_vhost0,iface=vhost-net,queues=2,client=1' -- -i --txd=1024 --rxd=1024 --txq=2 --rxq=2

  Expected Result:
  After the vhost-user is killed then re-launched, the virtio-net can connect back to vhost-user again.

  Actual Result:
  Vhost-user relaunch failed with continous log printed"VHOST_CONFIG: Processing VHOST_USER_SET_FEATURES failed.

  Analysis:
  This is a regression bug, bad commit: c6beefd674f
  When vhost-user quits, QEMU doesnot save acked features for each virtio-net after vhost-user quits. When vhost-user reconnects to QEMU, QEMU sends two different features(one is the true acked feature while the another is 0x40000000) to vhost-user successively which causing vhost-user exits abnormally.

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