Linux Virtualization - Jun 2013 - what should a virtio-mmio transport without a backend look like?

On 21 June 2013 17:02, Christopher Covington <cov at codeaurora.org>
wrote:
> Would using CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES enumeration
> instead of device tree be any easier?
My general view is that the kernel command line is
the user's to manipulate, and that QEMU shouldn't
touch it at all (just pass it through). (Conversely,
QEMU shouldn't require the user to specify odd
kernel command line arguments in order to make things
work.)

(Having talked with Grant Likely I now have a reasonable
plan for manipulating the dtb to add virtio nodes, so
I'm happy that that's the right way to go.)

As it happens, if you use the command line to specify
a virtio device it doesn't make the same complaint about
bad magic number as if you specify it via dtb, but that
should probably be fixed in the kernel :-)
> How does the back end know which devices to create?
Anything specified via -device on the QEMU command line
is created after the board model has initialized. When a
virtio-blk-backend (or whatever) is created it's automatically
plugged into the first available matching bus (in the same
way that if you create a PCI card via -device it plugs
into the first available PCI slot).

-- PMM

On Fri, 2013-06-21 at 17:41 +0100, Peter Maydell wrote:
> On 21 June 2013 17:02, Christopher Covington <cov at codeaurora.org>
wrote:
> > Would using CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES enumeration
> > instead of device tree be any easier?
> 
> My general view is that the kernel command line is
> the user's to manipulate, and that QEMU shouldn't
> touch it at all (just pass it through). (Conversely,
> QEMU shouldn't require the user to specify odd
> kernel command line arguments in order to make things
> work.)
I couldn't agree more. Command line is for the user and user only. And
the mention config option is optional ;-) (and, interestingly enough it
wasn't there in the original version of the driver).
> As it happens, if you use the command line to specify
> a virtio device it doesn't make the same complaint about
> bad magic number as if you specify it via dtb, but that
> should probably be fixed in the kernel :-)
I don't really see how this would be possible - the "complaining
code"
is just a normal platform device probe function. And whether specified
in the command line or in the tree, it's the same - platform - device.

Are you sure you haven't misspelled the parameter name or something in
the definition syntax? ;-)

Pawe?

On 21 June 2013 17:47, Pawel Moll <pawel.moll at arm.com>
wrote:
> On Fri, 2013-06-21 at 17:41 +0100, Peter Maydell wrote:
>> As it happens, if you use the command line to specify
>> a virtio device it doesn't make the same complaint about
>> bad magic number as if you specify it via dtb, but that
>> should probably be fixed in the kernel :-)
>
> I don't really see how this would be possible - the "complaining
code"
> is just a normal platform device probe function.
Sorry, you're correct and I misremembered. I just retested
with specifying via command line and it behaves the same
way as via dtb.

-- PMM