virtio_mmio.device parameter unknown as kernel config option is disabled

[João Mikos <j.eduardo@gmail.com>]

Dear Debian Kernel Team,

I am experimenting with running a Debian Buster microVM under the
Firecracker hypervisor, using a stock Debian kernel (converted to
vmlinux format) and an initrd.img file. When booting the microVM with
this setup, I receive the following error message inside the microVM,
which halts booting:

[ 2.136564] virtio_mmio: unknown parameter 'device' ignored

This is the command line passed by Firecracker to the kernel:

[ 0.000000] Command line: console=ttyS0 reboot=k panic=1 pci=off
root=/dev/vda rw virtio_mmio.device=4K@0xd0000000:5

I could trace this to the flag CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES
being disabled in the default Debian kernel config.

I have reconfigured the Debian Buster kernel with the flag
CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y, enabling the ".device" parameter
to the virtio_mmio module. I have then installed the newly compiled
kernel in another host and regenerated an initrd.img image with the
new virtio_mmio module.

When I try to boot the microVM again with the recompiled module
everything works as expected.

So my questions would be:

1) Is there any specific reason for this flag to be disabled in the
Debian kernel?
2) If the flag is disabled for no specific reason, is there any chance
for this flag to be enabled in future Debian versions, so the
virtio_mmio module recognizes the .device parameter by default?

Best regards,
João