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

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

On Mon, Sep 22, 2025 at 03:33:59PM +0200, Christian Speich wrote:
> On Mon, Sep 22, 2025 at 12:33:26PM +0100, Daniel P. Berrangé wrote:
> > 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.
> 
> I don't mind the 'x-'. And if that makes it clear, that this is used
> without any warrenty, sure!
> 
> However I'm not sure where the "risky" comes from. Initially confusion
> was given as reason.

I view it as "risky" in two ways

 - this device makes it very easy for a user to shoot themselves in
   the foot
 - we dont want to have to think about compatibility across QEMU
   releases in case there is something peculiar about a particular
   device type.

IMHO, adding the 'x-' prefix and disclaiming full support is sufficient
mitigation.

> Initially I thought about some kind of '--i-really-want-to-do-this'
> flag, but somehow I don't really see this device to bethis harmful
> to to warrent that big of a deterrent.

I agree.

With regards,
Daniel
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