Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 1/2] spapr: Add support for hwrng when available

[Alexander Graf <agraf@suse.de>]

On 11.09.15 02:46, David Gibson wrote:
> On Thu, Sep 10, 2015 at 02:13:26PM +0200, Alexander Graf wrote:
>>
>>
>>> Am 10.09.2015 um 14:03 schrieb Thomas Huth <thuth@redhat.com>:
>>>
>>>> On 10/09/15 12:40, David Gibson wrote:
>>>>> On Thu, Sep 10, 2015 at 09:33:21AM +0200, Thomas Huth wrote:
>>>>>> On 09/09/15 23:10, Thomas Huth wrote:
>>>>>> On 08/09/15 07:15, David Gibson wrote:
>>>>> ...
>>>>>>> At this point rather than just implementing them as discrete machine
>>>>>>> options, I suspect it will be more maintainable to split out the
>>>>>>> h-random implementation as a pseudo-device with its own qdev and so
>>>>>>> forth.  We already do similarly for the RTAS time of day functions
>>>>>>> (spapr-rtc).
>>>>>>
>>>>>> I gave that I try, but it does not work as expected. To be able to
>>>>>> specify the options, I'd need to instantiate this device with the
>>>>>> "-device" option, right? Something like:
>>>>>>
>>>>>>    -device spapr-rng,backend=rng0,usekvm=0
>>>>>>
>>>>>> Now this does not work when I use TYPE_SYS_BUS_DEVICE as parent class
>>>>>> like it is done for spapr-rtc, since the user apparently can not plug
>>>>>> device to this bus on machine spapr (you can also not plug an spapr-rtc
>>>>>> device this way!).
>>>>>>
>>>>>> The spapr-vlan, spapr-vty, etc. devices are TYPE_VIO_SPAPR_DEVICE, so I
>>>>>> also tried that instead, but then the rng device suddenly shows up under
>>>>>> /vdevice in the device tree - that's also not what we want, I guess.
>>>>>
>>>>> I did some more tests, and I think I can get this working with one small
>>>>> modification to spapr_vio.c
>>> ...
>>>>> i.e. when the dt_name has not been set, the device won't be added to the
>>>>> /vdevice device tree node. If that's acceptable, I'll continue with this
>>>>> approach.
>>>>
>>>> A bit hacky.
>>>>
>>>> I think it would be preferable to build it under SysBus by default,
>>>> like spapr-rtc.  Properties can be set on the device using -global (or
>>>> -set, but -global is easier).
>>>
>>> If anyhow possible, I'd prefere to use "-device" for this instead, because
>>>
>>> a) it's easier to use for the user, for example you can simply use
>>>   "-device spapr-rng,?" to get the list of properties - this
>>>   does not seem to work with spapr-rtc (it has a "date" property
>>>   which does not show up in the help text?)
>>>
>>> b) unlike the rtc device which is always instantiated, the rng
>>>   device is rather optional, so it is IMHO more intuitive if
>>>   created via the -device option.
>>>
>>> So I'd like to give it a try with the TYPE_VIO_SPAPR_DEVICE first ... if
>>> you then still don't like the patches at all, I can still rework them to
>>> use TYPE_SYS_BUS_DEVICE instead.
>>
>> Please don't use sysbus. If the vio device approach turns ugly,
>> create a new spapr hcall bus instead. We should have had that from
>> the beginning really.
> 
> Ok.. why?
> 
> It's a system (pseudo-)device that doesn't have any common bus
> infrastructure with anything else.  Isn't that what SysBus is for?

No, sysbus means "A device that has MMIO and/or PIO connected via a bus
I'm too lazy to model" really. These devices have neither.

Back in the days before QOM, sysbus was our lowest common denominator,
but now that we have TYPE_DEVICE and can branch off of that, we really
shouldn't abuse sysbus devices for things they aren't.


Alex