Re: virtio_mmio.device parameter unknown as kernel config option is disabled
On Fri, Mar 6, 2020 at 12:23 AM Noah Meyerhans <noahm@debian.org> wrote:
> Hi João. I have extensive experience with Firecracker, with both Debian
> and other distros as the guest OS. In general, the expectation with
> Firecracker is that it will not work with distro kernels out-of-the-box,
> but that it requires a custom kernel specifically configured for MicroVM
> deployments. Keep in mind that Firecracker originally didn't even
> provide initramfs support, meaning that all device and filesystem
> drivers needed to be statically linked into the kernel. Although
> Firecracker has since added initramfs support, its goal of minimal
> feature sets and extremely fast boot times still encourages you to use a
> customized kernel.
Hey Noah, thanks for the quick response and for the insights on
Firecracker. Indeed, the main reason why I resumed these experiments
were because of the recent addition of initrd support.
>
> > I could trace this to the flag CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES
> > being disabled in the default Debian kernel config.
> ...
> > 1) Is there any specific reason for this flag to be disabled in the
> > Debian kernel?
>
> It's not generally useful, so it hasn't been deemed appropriate for
> inclusion in the standard kernel builds. It'll be up to the kernel team
> to decide if supporting Firecracker with the default kernel is a
> sufficiently interesting use-case to warrant changing it. IMO, it is
> not.
Reflecting about what you said the default kernel might indeed not be
the best place to enable this, but maybe the cloud kernel could be a
good candidate instead. Official support for Firecracker might be a
stretch, especially due to the goals you've mentioned, but I am seeing
projects (e.g. Ignite) using Firecracker as a basis. Any such derived
project would be benefited by this change. But indeed, the kernel team
has the best view on whether they want to support (with all that
entails) any new flag or feature.
>
> > 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?
>
> You should open a wishlist severity bug against the src:linux package to
> officially request this change.
Thanks for the tip! I'll follow this direction and see what the team
thinks about it. Worst case scenario if anyone even stumbles upon a
similar issue the documentation of what to do is now here for
posterity.
Best regards,
João
On Fri, Mar 6, 2020 at 12:23 AM Noah Meyerhans <noahm@debian.org> wrote:
>
> On Thu, Mar 05, 2020 at 10:31:37PM +0100, João Mikos wrote:
> > 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:
>
> Hi João. I have extensive experience with Firecracker, with both Debian
> and other distros as the guest OS. In general, the expectation with
> Firecracker is that it will not work with distro kernels out-of-the-box,
> but that it requires a custom kernel specifically configured for MicroVM
> deployments. Keep in mind that Firecracker originally didn't even
> provide initramfs support, meaning that all device and filesystem
> drivers needed to be statically linked into the kernel. Although
> Firecracker has since added initramfs support, its goal of minimal
> feature sets and extremely fast boot times still encourages you to use a
> customized kernel.
>
> > I could trace this to the flag CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES
> > being disabled in the default Debian kernel config.
> ...
> > 1) Is there any specific reason for this flag to be disabled in the
> > Debian kernel?
>
> It's not generally useful, so it hasn't been deemed appropriate for
> inclusion in the standard kernel builds. It'll be up to the kernel team
> to decide if supporting Firecracker with the default kernel is a
> sufficiently interesting use-case to warrant changing it. IMO, it is
> not.
>
> > 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?
>
> You should open a wishlist severity bug against the src:linux package to
> officially request this change.
>
> noah
>
--
j.
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