[Bug 1546445] Re: support vhost user without specifying vhostforce

[Liang Chen]

** Also affects: cloud-archive
   Importance: Undecided
       Status: New

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Title:
  support vhost user without specifying vhostforce

Status in Ubuntu Cloud Archive:
  New
Status in QEMU:
  In Progress
Status in qemu package in Ubuntu:
  New
Status in qemu source package in Vivid:
  Won't Fix
Status in qemu source package in Wily:
  Fix Released

Bug description:
  [Impact]

   * vhost-user falls back to virtio-net which causes performance lose
  without specifying the vhostforce option. But it should be the default
  behavior for vhost-user, since guests using PMD doesn't support msi-x.

  [Test Case]

    create a vhost-user virtio backend without specifying the vhostforce option, i.e. -netdev type=vhost-user,id=mynet1,chardev=<char_dev_for_the_controll_channel>
    start the VM
    vhost-user is not enabled

  [Regression Potential]

   * none

  vhost user nic doesn't support non msi guests(like pxe stage) by default.
  Vhost user nic can't fall back to qemu like normal vhost net nic does. So we should
  enable it for non msi guests.

  The problem has been fix in qemu upstream  -
  http://git.qemu.org/?p=qemu.git;a=commitdiff;h=24f938a682d934b133863eb421aac33592f7a09e.
  And the patch needs to be backported to 1:2.2+dfsg-5expubuntu9.8 .

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