[Qemu-devel] Which functions of southbridges should be no-user? (was: [PATCH RFC 0/9] Clean up and fix no_user)

[Markus Armbruster <armbru@redhat.com>]

Andreas,

To go beyond RFC with this series, I need to explain why the IDE
controller functions of southbridges piix3-ide, piix3-ide-xen, piix4-ide
and via-ide cannot_instantiate_with_device_add_yet, or drop that.  I'd
appreciate your help.

Our modelling of PCI devices is weird, to put it politely.  One of many
weird things is that we don't distinguish between a function and a
complete device: our "function models" are actually PCI device models,
and can be used as such, even though they only exist as functions of a
multifunction device in the real world.  We permit collecting aribitrary
PCI devices into multifunction devices.

One instance of multifunction PCI devices are southbridges.  For
example, the ICH9 southbridge's PCI device 00:1F consists of ISA bridge
("ICH9 LPC"), IDE controller ("ich9-ahci"), SMB controller ("ICH9 SMB"),
and Thermal Subsystem (which we don't implement).  The PIIX3 southbridge
consists of ISA bridge ("PIIX3", IDE controller ("piix3-ide"), USB
controller ("piix3-usb-uhci", can be suppressed), and SMB controller
("PIIX4_PM", can be suppressed).

Some functions of southbridges still need to be wired up in code ("ICH9
LPC", "ICH9 SMB", "PIIX4_PM", "PIIX4", "PIIX3", "PIIX3-xen", see PATCH
5-6/9), thus cannot_instantiate_with_device_add_yet.

The IDE controller functions have always been
cannot_instantiate_with_device_add_yet, but it's not obvious to me why.

The other functions are available with device-add.  Users device-add'ing
them would of course be odd, but if it works...  I don't actually know
whether it works for all of them.

Should all southbridge functions be made unavailable with device-add for
consistency, at least for now?

Or should all functions be made available, except for the ones that
cannot possibly work with device-add?

If the latter, can you think of any specific reason why the IDE
controllers couldn't work?