[Rust-VMM] Crate for vhost-user device backend?

[Stefan Hajnoczi]

On Thu, Jan 17, 2019 at 11:00:27PM +0800, Liu, Jing2 wrote:
> On 1/17/2019 7:32 PM, Jiang Liu wrote:
> > > On Jan 17, 2019, at 7:30 PM, Dr. David Alan Gilbert <dgilbert at redhat.com> wrote:
> > > > > On Jan 17, 2019, at 6:58 PM, Marc-André Lureau <marcandre.lureau at gmail.com> wrote:
> > > > > On Thu, Jan 17, 2019 at 2:56 PM Jiang Liu <liuj97 at gmail.com <mailto:liuj97 at gmail.com>> wrote:
> What application will be the virtio backend on your scenario?

This is a technicality but an important one:

  vhost-user device backends are not the same as virtio device backends

vhost-user device backends subsets of virtio device backends.  For
example, they may lack some virtqueues, virtio configuration space, and
the device lifecycle is different.

The way that the vhost-user protocol works today, for better or for
worse, is that some of the virtio device backend still lives inside the
emulator (e.g. the virtio PCI transport which controls negotiation and
lifecycle).

This also means that the vhost-user protocol has messages and features
that have no equivalent in the virtio specification.

It's worth keeping the distinction in mind because confusing these two
things becomes problematic especially when the codebase supports both
virtio and vhost-user devices :).

Stefan
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