Re: Configuring onboard devices (was: Failing property setters + hardwired devices + -global = a bad day)

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

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.


ATB,

Mark.