Re: [PATCH] virtio: vhost-user-device: Make user creatable again

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Sep 19, 2025 at 04:07:19PM -0400, Michael S. Tsirkin wrote:
> On Fri, Sep 19, 2025 at 04:30:53PM +0200, Christian Speich wrote:
> > This removes the change introduced in [1] that prevents the use of
> > vhost-user-device and vhost-user-device-pci on unpatched QEMU builds.
> > 
> > [1]: 6275989647efb708f126eb4f880e593792301ed4
> > 
> > Signed-off-by: Christian Speich <c.speich@avm.de>
> > ---
> > vhost-user-device and vhost-user-device-pci started out as user
> > creatable devices. This was changed in [1] when the vhost-user-base was
> > introduced.
> > 
> > The reason given is to prevent user confusion. Searching qemu-discuss or
> > google for "vhost-user-device" I've seen no confused users.
> > 
> > Our use case is to provide wifi emulation using "vhost-user-device-pci",
> > which currently is working fine with the QEMU 9.0.2 present in Ubuntu
> > 24.04. With newer QEMU versions we now need to patch, distribute and
> > maintain our own QEMU packages, which is non-trivial.
> > 
> > So I want to propose lifting this restriction to make this feature
> > usable without a custom QEMU.
> > 
> > [1]: 6275989647efb708f126eb4f880e593792301ed4
> 
> The confusion is after someone reuses the ID you are claiming without
> telling anyone and then linux guests will start binding that driver to
> your device.
> 
> 
> We want people doing this kind of thing to *at a minimum*
> go ahead and register a device id with the virtio TC,
> but really to write and publish a spec.

Wanting people to register a device ID is a social problem and
we're trying to apply a technical hammer to it, which is rarely
an productive approach.

If we want to demonstrate that vhost-user-device is "risky", then
how about we rename it to have an 'x-' prefix and thus disclaim
any support for it, but none the less allow its use. Document it
as an experimental device, and if it breaks, users get to keep
both pieces.

It seems like it would be useful before any virtio spec submission
as a mechanism by which users can experiment to finese their ideas
prior to getting to the point of needing to make a spec proposal.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|