Re: [Qemu-devel] RFC: virtio-peer shared memory based peer communication device

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 21, 2015 at 02:32:10PM +0200, Jan Kiszka wrote:
> On 2015-09-21 14:13, Michael S. Tsirkin wrote:
> > On Fri, Sep 18, 2015 at 06:29:27PM +0200, Claudio Fontana wrote:
> >> Hello,
> >>
> >> this is a first RFC for virtio-peer 0.1, which is still very much a work in progress:
> >>
> >> https://github.com/hw-claudio/virtio-peer/wiki
> >>
> >> It is also available as PDF there, but the text is reproduced here for commenting:
> >>
> >> Peer shared memory communication device (virtio-peer)
> >>
> >> General Overview
> >>
> >> (I recommend looking at the PDF for some clarifying pictures)
> >>
> >> The Virtio Peer shared memory communication device (virtio-peer) is a
> >> virtual device which allows high performance low latency guest to
> >> guest communication. It uses a new queue extension feature tentatively
> >> called VIRTIO_F_WINDOW which indicates that descriptor tables,
> >> available and used rings and Queue Data reside in physical memory
> >> ranges called Windows, each identified with an unique identifier
> >> called WindowID.
> > 
> > So if I had to summarize the difference from regular virtio,
> > I'd say the main one is that this uses window id + offset
> > instead of the physical address.
> > 
> > 
> > My question is - why do it?
> > 
> > All windows are in memory space, are they not?
> > 
> > How about guest using full physical addresses,
> > and hypervisor sending the window physical address
> > to VM2?
> > 
> > VM2 can uses that to find both window id and offset.
> > 
> > 
> > This way at least VM1 can use regular virtio without changes.
> 
> What would be the value of having different drivers in VM1 and VM2,
> specifically if both run Linux?
> 
> Jan

It's common to have a VM act as a switch between others.

In this setup, there's value in being able to support existing guests as
endpoints, with new drivers only required for the switch.

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