Re: Should linuxkpi linux_file_ioctl support flexible array copy?

On Sun, Feb 15, 2026 at 03:38:59AM +0000, Bjoern A. Zeeb wrote:
>
> I was mostly curious to find out if/why this would be LinuxKPI code, or
why
> we would need the LinuxKPI interface/structures for this?

FreeBSD's modern graphics stack (`drm-kmod`) is essentially a port of
the Linux DRM subsystem running on top of LinuxKPI.  So the interfaces
and data structures of dma-buf subsystem in FreeBSD exposed to the GPU
drivers are strictly Linux-compatible. So as a dmabuf exporter, udmabuf is
inevitably relying some Linuxkpi.

However, I try to not relying any other Linuxkpi except the dma-buf
interface in internal implementation of udmabuf.

I think that keeping the udmabuf ABI untouched is a nice-to-have, which
makes modifying userspace programs easier. But since neither the native
FreeBSD KPI nor the LinuxKPI currently supports flexible arrays
effectively, I decide to modify the ABI.


> I am currently assuming that the main benefit would be if using some
> virtio-drm driver and wanting to display the guest FB locally on the host
> or in a GPU VF/PF setup rather than streaming it away?  But there's a
> missing piece to the puzzle for me still.
>

I think your assumption is spot on regarding the benefit!


> That said, it all sounds totally exciting :)

Thanks for the encouragement!

Best regards,
Zishun Yi