Re: [RFC] Proposal for a QEMU video subsystem for cameras and webcams

[Thomas Huth <thuth@redhat.com>]

On 10/03/2025 17.51, Alex Bennée wrote:
> David Milosevic <david.milosevic@9elements.com> writes:
> 
>> Dear QEMU Developers,
>>
>> I would like to propose the development of a video subsystem in QEMU, with the initial
>> implementation focusing on UVC video device emulation and support for multiple
>> backends, including V4L2, GStreamer, and libcamera.
>>
>> This work is already in progress at 9elements, and we would like to upstream it.
>>
>> == Motivation
>>
>> Currently, USB pass-through is the only way to make video devices available to guests, which
>>
>>      - excludes non-USB cameras (e.g., MIPI)
>>      - performs poorly with high-resolution cameras
>>      - does not work with USB 3.0 video devices (Issue #1613)
>>
>> == Proposal
>>
>> We aim to introduce a video subsystem in QEMU that allows for the implementation of various
>> video devices, similar to how QEMU handles audio. The first device implementation will be
>> UVC (USB Video Class) device emulation, with support for multiple backends. Future extensions
>> could include virtio-video or other PCI-based video devices.
> 
> Are you aware of virtio-media? It was an alternative proposal to
> virtio-video which effectively becomes an encapsulation of v4l to the
> guest.

... but USB video would also be nice, wouldn't it? That could enable guests 
to use webcams without needing additional virtio drivers for it, I think?

>> Supported backends:
>>
>>      - Video4Linux (V4L2)
>>      - GStreamer
>>      - libcamera
>>
>> == Example: V4L2 Backend
>>
>> Once implemented, a typical QEMU command line for using a V4L2 backend would look like this
>>
>>      ./build/qemu-system-x86_64 \
>>          -device qemu-xhci \
>>          -videodev v4l2,id=cam0,device=/dev/video0 \

Just a quick comment here: QEMU tries often to avoid to introduce new 
top-level command line switches nowadays. Would it be possible to use 
"-object" for this instead, like it is e.g. done with the memory backends or 
rng backends?

  Thomas


>>          -device usb-video,videodev=cam0
>>
>> This sets up a UVC emulated device in the guest, using /dev/video0 from the
>> host via the V4L2 backend.
>>
>> == Next Steps
>>
>> We welcome feedback on design considerations and integration approaches. Let us know
>> if there are existing discussions or preferred directions for this work.
>>
>> Best regards,
>>
>> David Milosevic
>> Firmware Developer
>> 9elements
>