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Re: [Qemu-devel] [PATCH for-3.2 01/11] vhost-user: define conventions fo

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH for-3.2 01/11] vhost-user: define conventions for vhost-user backends
Date: Tue, 11 Dec 2018 13:56:04 -0500

On Tue, Dec 11, 2018 at 09:29:44AM +0000, Daniel P. Berrangé wrote:
> On Tue, Dec 11, 2018 at 08:42:41AM +0100, Hoffmann, Gerd wrote:
> >   Hi,
> > 
> > > Right. The main issue is that we need to make sure only
> > > in-tree devices are supported.
> > 
> > Well, that is under debate right now, see:
> > https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg04912.html
> I've previously been against the idea of external plugins for QEMU,
> however, that was when the plugin was something that would be dlopen'd
> by QEMU. That would cause our internal ABI to be exposed to 3rd parties
> which is highly undesirable, even if they were open source to comply
> with the license needs.
> When the plugin is a completely isolated process communicating with a
> well defined protocol, it is not placing a significant burden on the
> QEMU developers' ongoing maintainence, nor has problems with license
> compliance. The main problem would come from debugging the combined
> system as the external process is essentially a black box from QEMU's
> POV. Downstream OS vendors are free to place restrictions on which
> backend processes they'd be willing to support with QEMU, and upstream
> is under no obligation to debug stuff beyond the QEMU boundary.
> We have already accepted that tradeoff with networking by supporting
> vhost-user and have externals impls like DPDK, so I don't see a
> compelling reason to try to restrict it for other vhost-user backends.

OK seems to be more or less a rough concensus then.

I wonder what's the approach wrt migration though.

Even the compatibility story about vhost-user isn't
great, I would like to see something solid before
we allow that.

Are we happy to just block live migration?
For sure that's already the case with VFIO.

> > > vhost-user by design
> > > is for out of tree users. It needn't be hard,
> > > maybe it's enough to just make qemu launch these processes
> > > as opposed to connecting to them on command line.
> > 
> > Not sure this is a good idea, with security being one of the motivating
> > factors to move device emulation to other processes.  When libvirt
> > launches the processes it can place them in separate sandboxes ...
> Yep, libvirt already turns on seccomp policies which forbid QEMU from
> forking/execing anything, and we have no desire to go backwards here.
> Any external processes have to be launched by libvirt ahead of time.
> Regards,
> Daniel
> -- 
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