Re: [Qemu-devel] [virtio-comment] [PATCH] *** Vhost-pci RFC v2 ***

[Wang, Wei W]

On Monday, August 29, 2016 11:25 PM, Stefan Hajnoczi wrote:
> To: Wang, Wei W <address@hidden>
> Cc: address@hidden; address@hidden; virtio- 
> address@hidden; address@hidden; address@hidden
> Subject: Re: [virtio-comment] [PATCH] *** Vhost-pci RFC v2 ***
> 
> On Mon, Jun 27, 2016 at 02:01:24AM +0000, Wang, Wei W wrote:
> > On Sun 6/19/2016 10:14 PM, Wei Wang wrote:
> > > This RFC proposes a design of vhost-pci, which is a new virtio device 
> > > type.
> > > The vhost-pci device is used for inter-VM communication.
> > >
> > > Changes in v2:
> > > 1. changed the vhost-pci driver to use a controlq to send acknowledgement
> > >    messages to the vhost-pci server rather than writing to the device
> > >    configuration space;
> > >
> > > 2. re-organized all the data structures and the description 
> > > layout;
> > >
> > > 3. removed the VHOST_PCI_CONTROLQ_UPDATE_DONE socket message,
> which
> > > is redundant;
> > >
> > > 4. added a message sequence number to the msg info structure to 
> > > identify socket
> > >    messages, and the socket message exchange does not need to be 
> > > blocking;
> > >
> > > 5. changed to used uuid to identify each VM rather than using the 
> > > QEMU
> process
> > >    id
> > >
> >
> > One more point should be added is that the server needs to send 
> > periodic socket messages to check if the driver VM is still alive. I 
> > will add this message support in next version.  (*v2-AR1*)
> 
> Either the driver VM could go down or the device VM (server) could go 
> down.  In both cases there must be a way to handle the situation.
> 
> If the server VM goes down it should be possible for the driver VM to 
> resume either via hotplug of a new device or through messages 
> reinitializing the dead device when the server VM restarts.

I got feedbacks from people that the name of device VM and driver VM are 
difficult to remember. Can we use client (or frontend) VM and server (or 
backend) VM in the discussion? I think that would sound more straightforward :)

Here are the two cases:
 
Case 1: When the client VM powers off, the server VM will notice that the 
connection is closed (the client calls the socket close() function, which 
notifies the server about the disconnection). Then the server will need to 
remove the vhost-pci device for that client VM. When the client VM boots up and 
connects to the server again, the server VM re-establishes the inter-VM 
communication channel (i.e. creating a new vhost-pci device and hot-plugging it 
to the server VM).

Case 2: When the server VM powers off, the client doesn't need to do anything. 
We can provide a way in QEMU monitor to re-establish the connection. So, when 
the server boots up again, the admin can let a client connect to the server via 
the client side QEMU monitor.

Best,
Wei