Re: [Qemu-trivial] [Qemu-devel] [PATCH] hw/input/pckbd: The i8042 device should not be user_creatable

[Thomas Huth]

On 04/04/2019 18.30, Markus Armbruster wrote:
> Thomas Huth <address@hidden> writes:
> 
>> On 04/04/2019 15.29, Philippe Mathieu-Daudé wrote:
>>> On 4/4/19 12:07 PM, Paolo Bonzini wrote:
>>>> On 04/04/19 09:14, Thomas Huth wrote:
>>>>> The i8042 PS/2 controller is part of the chipset on the motherboard.
>>>>> It is instantiated by the machine init code, and it does not make sense
>>>>> to allow the user to plug an additional i8042 in any of the free ISA 
>>>>> slots.
>>>>> Thus let's mark the device with user_creatable = false.
>>>>>
>>>>> Signed-off-by: Thomas Huth <address@hidden>
>>>>> ---
>>>>>  hw/input/pckbd.c | 2 ++
>>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> user_creatable is not for devices that are not pluggable in real life;
>>>> it is for devices that crash QEMU (!) or always fail if plugged by the 
>>>> user.
>>
>> ... hmm, but presenting devices to the user that are clearly not
>> intended for direct use is also not very nice, is it?
> 
> Maybe, but hiding them should be separate from marking devices that
> still defeat device_add.
> 
> In the ideal world, we'd be able to start with an empty board, then
> build a machine with by wiring together devices.  Say like use
> device_add to create and plug into parent bus, qom-set link properties
> to create additional wires.

I agree with you on the vision of creating a machine dynamically - but I
doubt that we will ever do it with the "high level" -device option or
device_add HMP/QMP command. You'd likely rather need to introduce a new
interface / commands for this anyway, which then simply ignore the
normal user_creatable flag.

> Plenty of devices fail at the device_add stage, or require additional
> wiring by code.  These are marked not user_creatable.

Sure, I think we all agree on these kind of devices.

But what about these devices which obviously don't crash QEMU, but are
completely useless with "-device", since they can not be used in any
meaningful way? Presenting them to the average user e.g. in the "-device
help" output is somewhat gruesome, isn't it?

>>>> So the question to ask is: would it make sense, and especially work, to
>>>> add an i8042 to machines that do have an ISA bridge (for example the 
>>>> Alpha?)
> 
> Scratch the "would it make sense" part, keep the "would it work" part.

I don't see a way how this could really work (in the sense of being
usable, not in the sense of not crashing QEMU). The device is only
expected to be there on certain chipssets, the guest does not have a
possibility to detect this device on systems that do not feature one of
these chipssets. So you get a device in such a case that simply does
nothing. How is that useful for you?

 Thomas