Re: Configuring onboard devices

[Markus Armbruster <armbru@redhat.com>]

Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:

> On 30/04/2020 15:11, Markus Armbruster wrote:
>
>> Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:
>> 
>>> On 30/04/2020 11:03, Markus Armbruster wrote:
>>>> Peter Maydell <peter.maydell@linaro.org> writes:
>>>>
>>>>> On Thu, 30 Apr 2020 at 08:09, Markus Armbruster <armbru@redhat.com> wrote:
>>>>>> Our means to configure onboard devices are weak.  We sidestepped this
>>>>>> for isa-fdc by taking it off the board, and thus make -device work.
>>>>>
>>>>> This seems to be a general dynamic: the x86 pc machine works
>>>>> via -device options (or is changed so it can work that way);
>>>>> and then people propose dropping/deprecating/etc the config
>>>>> options that work with onboard devices, without providing
>>>>> clear solutions/instructions on how the command line needs
>>>>> to change/etc for the mass of boards which are not the x86
>>>>> pc machine and which do have a lot of onboard devices which
>>>>> can't be handled via -device.
>>>>>
>>>>> So my gut reaction to the "we should deprecate -global"
>>>>> suggestions in this thread was a bit "here we go again"...
>>>>> What works for x86 or even "what is sufficient for libvirt"
>>>>> doesn't necessarily cover all the cases.
>>>>
>>>> Such shortsighted proposals have been made, but don't think it's what
>>>> we're doing here.
>>>>
>>>> You're 100% right in that we do need to configure onboard devices.
>>>> -global is a terrible way to do it, though: it applies to *all* devices
>>>> of a kind.  What if the board has more than one?  What if the can add
>>>> more?
>>>>
>>>> Taking onboard devices off the board can occasionally sidestep the
>>>> issue.  For isa-fdc, we genuinely *wanted* to take the damn thing off,
>>>> because all it did for most users was provide them with VENOM.  Not
>>>> needing -global for it anymore was just a nice bonus.
>>>>
>>>> Taking onboard devices off just to reduce the device configuration
>>>> problem to a solved one, namely -device, may be tempting (it was to me),
>>>> but it's too intrusive to be practical at scale.
>>>>
>>>> Adding machine properties that alias onboard device properties is less
>>>> intrusive.  The ones I added were still a lot of work.
>>>>
>>>> Configuring onboard devices via machine properties restricts property
>>>> access to the ones we added to the machine.  This differs from pluggable
>>>> devices, where users can access all properties.
>>>>
>>>> Any better ideas for letting users configure onboard devices?
>>>
>>> Is it possible to let machine owners add alias properties to the machine object
>>> referencing in-built devices? I could then instantiate my on-board nic in the machine
>>> init() function, and then use object_property_add_alias() to add a "nic0" alias on
>>> the machine that can be used to wire it up to a netdev using the command line.
>> 
>> Have a look at hw/arm/virt.c's virt_flash_create(), from commit
>> e0561e60f1 "hw/arm/virt: Support firmware configuration with -blockdev".
>> It adds machine properties "pflash0" and "pflash1" as aliases for the
>> onboard flash memory devices' property "drive".
>> 
>> Does this answer your question?
>
> Ah I see now, these aliases are for individual properties rather than objects. What I
> was trying to ask was if it were possible to have something like this:
>
> /machine (SS-5-machine)
>   /builtin
>     /nic0 -> link to "lance" device
>
> Here nic0 is an alias "published" by the maintainer of the SS-5 machine which is
> configured in the machine init() function using object_property_add_link() or a
> suitable wrapper. Users can then configure these builtin devices from the command
> line using your -machine nic0.netdev=my-netdev-id syntax or similar.

Got it now, thanks!

> Having the default devices under /builtin or other known QOM path would enable
> builtin devices to be easily enumerated programatically and/or from the command line
> as required.

There are three standard containers under /machine/:

* /machine/peripheral/

  Devices with a user-specified ID go here, as /machine/peripheral/ID.
  User-specified means -device or device_add.

  /machine/peripheral/ID is effectively a stable interface.  It's just
  underdocumented (undocumented?).

  To be useful, the stuff below ID/ needed to be stable and documented,
  too.

* /machine/peripheral-anon/

  Same, but user elected not to give an ID.
  /machine/peripheral-anon/device[N], where N counts up from zero in
  creation order.

  N is obviously not stable, but this is a problem of the user's making.
  If you want to refer to a device, give it an ID.

* /machine/unattached/

  The orphanage.  When a device has no parent when its realized, it gets
  put here, as /machine/unattached/device[N], where N counts up from
  zero in realization order.

  N is obviously not stable, and this time we can't blame the
  victim^Wuser.  You can search for devices of a certain type.
  Sometimes that's good enough.

  All the onboard devices are here, and much more.  We've fathered a lot
  of unloved red-headed children, it seems...

  Some of the "much more" is due to sloppy modelling, i.e. neglecting to
  set the proper parent.

  I figure we could put onboard devices in a nicer place, with nicer
  names.  Need a convention for the place and the names, then make board
  code conform to it.