Re: [Qemu-devel] [PATCH v1 1/1] vl.c: Allow sysbus devices to be attached via commandline

[Markus Armbruster]

Alistair Francis <address@hidden> writes:

> On Wed, Apr 9, 2014 at 11:28 AM, Peter Crosthwaite
> <address@hidden> wrote:
>> On Wed, Mar 26, 2014 at 10:47 AM, Alistair Francis
>> <address@hidden> wrote:
>>> This patch introduces a new command line argument that allows
>>> sysbus devices to be attached via the command line.
>>>
>>> This allows devices to be added after the machine init but
>>> before anything is booted
>>>
>>> The new argument is -sysbusdev
>>>
>>> A new argument is being used to avoid confusion and user
>>> errors that would appear if the same command for hot-plugging
>>> was used instead. This could be changed to use -device and
>>> specify sysbus by using something like 'bus=sysbus' if that
>>> is preferred
>>>
>>
>> Can you be more specific about the confusion issue you are seeing? If
>> you went for -device bus=root (meaning the default singleton root
>> sysbus), where are the possible issues?
>>
>
> Using -device would be possible (and easy to implement). The reason I
> started with
> a different argument name was just to avoid confusion and to make sure
> I didn't interfere with hot-pluggable devices. I still feel that separating
> hot-pluggable devices from bus devices is a good idea. They are initialised
> in slightly different parts of the code (although this wouldn't
> be hard to fix). The name 'sysbusdev' is confusing though, as it doesn't
> have to connect to the sysbus.

-device / device_add already covers both hot plug and cold plug.  In
fact, only device_add can hot plug; all -device ever does is cold plug.

Plugging in a device involves wiring it up.  Hot plug additionally
involves communication with guest software, but let's ignore that for
now.

-device uses generic wiring code, guided by configuration: property
"bus".  This suffices for straightforward cases like PCI and USB
devices.  It doesn't for sysbus devices, because these may require
arbitrarily complex wirings.  Therefore, sysbus devices are unavailable
with -device; see commit 837d371.

What we need is a way for configuration to guide more general wiring.
Perhaps -device can be extended.  If that turns out to be impractical,
we need something more expressive that also covers everything -device
does now, and can cover "everything" eventually.  Andreas may have ideas
here.

What we don't need, in my opinion, is more special-case options :)

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