Re: [Qemu-devel] [PATCH RFC 00/14] vhost-user: shutdown and reconnection

[Michael S. Tsirkin]

On Wed, Sep 09, 2015 at 01:09:52AM +0200, address@hidden wrote:
> From: Marc-André Lureau <address@hidden>
> 
> In a previous series "Add feature to start QEMU without vhost-user
> backend", Tetsuya Mukawa proposed to allow the vhost-user backend to
> disconnect and reconnect. However, Michael Tsirkin pointed out that
> you can't do that without extra care, because the guest and hypervisor
> don't know the slave ring manipulation state, there might be pending
> replies for example that could be lost, and suggested to reset the
> guest queues, but this requires kernel changes, and it may have to
> clear the ring and lose queued packets.
> 
> The following series starts from the idea that the slave can request a
> "managed" shutdown instead and later recover (I guess the use case for
> this is to allow for example to update static dispatching/filter rules
> etc)

I'm still not sure users actually need this.  I am inclined to think we
should teach guests to respond to NEED_RESET status. Then to handle
disconnect, we would
- deactivate the disconnected backend
- stop VM, and wait for a reconnect
- set NEED_RESET status, and re-activate the backend
  after guest reset

-- 
MST