Re: [Qemu-devel] [PATCH] Vmchannel PCI device.

[Daniel P. Berrange]

On Sun, Dec 14, 2008 at 01:15:42PM -0600, Anthony Liguori wrote:
> Gleb Natapov wrote:
> >On Sun, Dec 14, 2008 at 02:28:23PM +0200, Blue Swirl wrote:
> >  
> >>On 12/14/08, Gleb Natapov <address@hidden> wrote:
> >>    
> >>>There is a need for communication channel between host and various
> >>> agents that are running inside a VM guest. The channel will be used
> >>> for statistic gathering, logging, cut & paste, host screen resolution
> >>> changes notification, guest configuration etc.
> >>>      
> >>Isn't this exactly what the firmware configuration device was supposed
> >>to be used for? In the list of use cases you gave, I don't see
> >>anything that could not be done with it.
> >>
> >>    
> >The requirement for firmware configuration interface was different. We
> >wanted something simple that we can use as early as possible in cpu init
> >code and performance was not considered at all. Obviously PCI device 
> >doesn't
> >fit for this. We don't want to write PCI driver inside a BIOS and PCI
> >initialization is too late in HW initialization sequence.
> >
> >The requirement for vmchannel was that it should allow a guest
> >to communicate with external (to qemu) process and with reasonable
> >performance too. 
> 
> This is not a requirement that I think is important.  It's only a 
> requirement for you because you have closed code that you want to 
> implement the backend with.  I would personally be more interested in 
> vmchannel backends in QEMU and I think there will be a lot of them.

One non-QEMU backend I can see being implemented is a DBus daemon,
providing a simple bus for RPC calls between guests & host. Or on
a similar theme, perhaps a QPid message broker in the host OS. Yet
another backend is a clustering service providing a virtual fence
device to VMs. All of these would live outside QEMU, and as such
exposing the backend using the character device infrastructure 
is a natural fit.

Daniel
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