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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, May 28, 2013 at 08:17:42PM +0300, Michael S. Tsirkin wrote:
> On Tue, May 28, 2013 at 12:00:38PM -0500, Anthony Liguori wrote:
> > Julian Stecklina <jsteckli@os.inf.tu-dresden.de> writes:
> > 
> > > 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.
> > 
> > We aren't going to support any interface that enables out of tree
> > devices.  This is just plugins in a different form with even more
> > downsides.  You cannot easily keep track of dirty info, the guest
> > physical address translation to host is difficult to keep in sync
> > (imagine the complexity of memory hotplug).
> > 
> > Basically, it's easy to hack up but extremely hard to do something that
> > works correctly overall.
> > 
> > There isn't a compelling reason to implement something like this other
> > than avoiding getting code into QEMU.  Best to just submit your device
> > to QEMU for inclusion.
> > 
> > If you want to avoid copying in a vswitch, better to use something like
> > vmsplice as I outlined in another thread.
> > 
> > > 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?
> > 
> > I don't see any compelling reason to do something like this.  It's
> > jumping through a tremendous number of hoops to avoid putting code that
> > belongs in QEMU in tree.
> > 
> > Regards,
> > 
> > Anthony Liguori
> > 
> > >
> > > Julian
> 
> OTOH an in-tree device that runs in a separate process would
> be useful e.g. for security.
> For example, we could limit a virtio-net device process
> to only access tap and vhost files.

For tap or vhost files only this is good for security.  I'm not sure it
has many advantages over a QEMU process under SELinux though.

Obviously when the switch process has shared memory access to multiple
guests' RAM, the security is worse than a QEMU process solution but
better than a vhost kernel solution.

So the security story is not a clear win.

Stefan