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

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> ---
>  docs/system/devices/vhost-user.rst | 10 ----------
>  hw/virtio/vhost-user-device-pci.c  |  3 ---
>  hw/virtio/vhost-user-device.c      |  3 ---
>  3 files changed, 16 deletions(-)
> 
> diff --git a/docs/system/devices/vhost-user.rst b/docs/system/devices/vhost-user.rst
> index 35259d8ec7c666aa0c56497b8261f48d77216ad5..2d130f9767dbb1cbb85cef43c63dc9a8d7b30d4a 100644
> --- a/docs/system/devices/vhost-user.rst
> +++ b/docs/system/devices/vhost-user.rst
> @@ -73,16 +73,6 @@ all the required parameters including:
>    - The ``num_vqs`` it needs and their ``vq_size``
>    - The ``config_size`` if needed
>  
> -.. note::
> -  To prevent user confusion you cannot currently instantiate
> -  vhost-user-device without first patching out::
> -
> -    /* Reason: stop inexperienced users confusing themselves */
> -    dc->user_creatable = false;
> -
> -  in ``vhost-user-device.c`` and ``vhost-user-device-pci.c`` file and
> -  rebuilding.
> -
>  vhost-user daemon
>  =================
>  
> diff --git a/hw/virtio/vhost-user-device-pci.c b/hw/virtio/vhost-user-device-pci.c
> index f10bac874e78429c633752a4ce9db28385b3bb07..c76a856c9b9a67d941a93929244216658ff2a156 100644
> --- a/hw/virtio/vhost-user-device-pci.c
> +++ b/hw/virtio/vhost-user-device-pci.c
> @@ -38,9 +38,6 @@ static void vhost_user_device_pci_class_init(ObjectClass *klass,
>      VirtioPCIClass *k = VIRTIO_PCI_CLASS(klass);
>      PCIDeviceClass *pcidev_k = PCI_DEVICE_CLASS(klass);
>  
> -    /* Reason: stop users confusing themselves */
> -    dc->user_creatable = false;
> -
>      k->realize = vhost_user_device_pci_realize;
>      set_bit(DEVICE_CATEGORY_INPUT, dc->categories);
>      pcidev_k->vendor_id = PCI_VENDOR_ID_REDHAT_QUMRANET;
> diff --git a/hw/virtio/vhost-user-device.c b/hw/virtio/vhost-user-device.c
> index 3939bdf755222a281da8ca22243d7d4f16990a66..1bea496afd0137ba9b42009f6252acf6800528d1 100644
> --- a/hw/virtio/vhost-user-device.c
> +++ b/hw/virtio/vhost-user-device.c
> @@ -41,9 +41,6 @@ static void vud_class_init(ObjectClass *klass, const void *data)
>  {
>      DeviceClass *dc = DEVICE_CLASS(klass);
>  
> -    /* Reason: stop inexperienced users confusing themselves */
> -    dc->user_creatable = false;
> -
>      device_class_set_props(dc, vud_properties);
>      dc->vmsd = &vud_vmstate;
>      set_bit(DEVICE_CATEGORY_INPUT, dc->categories);
> 
> ---
> base-commit: e7c1e8043a69c5a8efa39d4f9d111f7c72c076e6
> change-id: 20250919-vhost-user-device-creatable-b7f9b7b5bfb2
> 
> Best regards,
> -- 
> Christian Speich <c.speich@avm.de>