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

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Sep 11, 2015 at 11:43:02AM +0200, Alexander Graf wrote:
> 
> 
> 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.

Oh.

So.. where is one supposed to find that out?

> 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.

So what actually is the root of the qdev tree then?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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