Re: [PATCH-for-11.0 v6 08/13] hw/sparc64/sun4u: Use fw_cfg_init_io_nodma()

[Zhao Liu <zhao1.liu@intel.com>]

> > > The reason that the fw_cfg device lives under ebus on sun4u is because the
> > > ebus device is effectively a PCI-ISA bridge, and the fw_cfg port is mapped
> > > into I/O address space along with other ISA devices. I'm not sure that
> > > setting the parent to the machine is the right thing to do here.
> > 
> > Thanks Philippe and Mark, IIUC, ebus doesn't have the hotplug use case,
> > so it lives long enough like the sun4u/v machine, therefore replacing
> > the parent object "ebus" with machine is safe.
> 
> It's safe, but it still doesn't make sense for sun4u/v because there is no
> machine-level I/O address space as per x86. It really does exist as a
> separate legacy bus under a PCI bridge.

I tend to agree with Philippe's perspective - since it's per-Guest,
setting the parent as machine seems appropriate.

> That's correct, ebus does not support hotplug.
> 
> > Adding a "parent" argument to the generic interface fw_cfg_init_io_nodma()
> > doesn't seem necessary at the moment, since using the default machine as
> > parent seems enough to meet all current needs in QEMU.
> > 
> > What do you think?
> 
> My preference would be add to add the parent argument as it's easy to do,
> and doesn't attempt to enforce x86-type constraints on other architectures.

I think it's not a x86-specific issue. The parent parameter's role is
lifecycle management - if we allowed custom parents, we'd have to
account for various complications that could result from early parent
destruction, so it's not much easy IMO.

Thanks,
Zhao