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

[Peter Crosthwaite]

cc Alex

On Thu, Apr 10, 2014 at 10:54 AM, Alistair Francis
<address@hidden> wrote:
> On Wed, Apr 9, 2014 at 6:02 PM, Markus Armbruster <address@hidden> wrote:
>> 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.
>
> I have managed to extended -device to allow QOM devices to be attached
> to the system bus.
> I removed the -sysbusdev argument and am now back to just using
> "-device" where the
> user can specify which bus to connect to.
> This allows entire machines to be built from command line arguments
> using "-M none".
>
> I'm just tidying up the patch now
>
>>
>> What we don't need, in my opinion, is more special-case options :)
>>
>> [...]
>>
>