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

["Wang, Wei W" <wei.w.wang@intel.com>]

On 09/02/2016 09:27 PM, Stefan Hajnoczi wrote:
> On Fri, Sep 02, 2016 at 12:27:25AM +0800, Wei Wang wrote:
> > On 09/01/2016 12:07 AM, Stefan Hajnoczi wrote:
> > > On Tue, Aug 30, 2016 at 10:08:01AM +0000, Wang, Wei W wrote:
> > > > On Monday, August 29, 2016 11:25 PM, Stefan Hajnoczi wrote:
> > > > > To: Wang, Wei W <wei.w.wang@intel.com>
> > > > > Cc: kvm@vger.kernel.org; qemu-devel@nongnu.org; virtio- 
> > > > > comment@lists.oasis-open.org; mst@redhat.com; 
> > > > > pbonzini@redhat.com
> > > > > 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 :)
> > > We discussed this in a previous email thread.
> > >
> > > Device and driver are the terms used by the virtio spec.  Anyone 
> > > dealing with vhost-pci design must be familiar with the virtio spec.
> > >
> > > I don't see how using the terminology consistently can be 
> > > confusing, unless these people haven't looked at the virtio spec.  
> > > In that case they have no business with working on vhost-pci 
> > > because virtio is a prerequisite :).
> > >
> > > Stefan
> > I don't disagree :)
> > But "frontend/backend" is also commonly used in descriptions in 
> > virtio related stuff, and it seems that more people like it. It's 
> > also easier to describe some components in the design (e.g. a 
> > backend functionality like vhost-pci-net). I am not sure if you guys are also OK with it.
> 
> If you want to use frontend/backend I don't mind.  It seems clear to me.

Thanks Stefan. 

Marc-André and I just got different thoughts about a design direction. I prefer to have all the frontend virtio devices (net, scsi, console etc.) from the same VM to be supported by one backend vhost-pci device (N-1), while Marc-André prefers to have each frontend virtio device be supported by a backend vhost-pci device (N-N). 

If possible, hope you, Michael or other more people could also join our review and discussion to finalize the design. Thanks.

Best,
Wei