Re: [Qemu-devel] snabbswitch integration with QEMU for userspace ethernet I/O

[Julian Stecklina <jsteckli@os.inf.tu-dresden.de>]

On 05/28/2013 12:10 PM, Luke Gorrie wrote:
> On 27 May 2013 11:34, Stefan Hajnoczi <stefanha@redhat.com
> <mailto:stefanha@redhat.com>> wrote:
> 
>     vhost_net is about connecting the a virtio-net speaking process to a
>     tun-like device.  The problem you are trying to solve is connecting a
>     virtio-net speaking process to Snabb Switch.
> 
> 
> Yep!

Since I am on a similar path as Luke, let me share another idea.

What about extending qemu in a way to allow PCI device models to be
implemented in another process. This is not as hard as it may sound.
qemu would open a domain socket to this process and map VM memory over
to the other side. This can be accomplished by having file descriptors
in qemu to VM memory (reusing -mem-path code) and passing those over the
domain socket. The other side can then just mmap them. The socket would
also be used for configuration and I/O by the guest on the PCI
I/O/memory regions. You could also use this to do IRQs or use eventfds,
whatever works better.

To have a zero copy userspace switch, the switch would offer virtio-net
devices to any qemu that wants to connect to it and implement the
complete device logic itself. Since it has access to all guest memory,
it can just do memcpy for packet data. Of course, this only works for
64-bit systems, because you need vast amounts of virtual address space.
In my experience, doing this in userspace is _way less painful_.

If you can get away with polling in the switch the overhead of doing all
this in userspace is zero. And as long as you can rate-limit explicit
notifications over the socket even that overhead should be okay.

Opinions?

Julian