Re: [Qemu-devel] [PATCH] spapr-pci: enable adding PHB via -device

[Markus Armbruster <armbru@redhat.com>]

Alexey Kardashevskiy <aik@ozlabs.ru> writes:

> Recent changes introduced cannot_instantiate_with_device_add_yet
> and removed capability of adding yet another PCI host bridge via
> command line for SPAPR platform (POWERPC64 server).

Specifically:

commit 837d37167dc446af8a91189108b363c04609e296
Author: Markus Armbruster <armbru@redhat.com>
Date:   Thu Nov 28 17:26:55 2013 +0100

    sysbus: Set cannot_instantiate_with_device_add_yet
    
    device_add plugs devices into suitable bus.  For "real" buses, that
    actually connects the device.  For sysbus, the connections need to be
    made separately, and device_add can't do that.  The device would be
    left unconnected, and could not possibly work.
    
    Quite a few, but not all sysbus devices already set
    cannot_instantiate_with_device_add_yet in their class init function.
    
    Set it in their abstract base's class init function
    sysbus_device_class_init(), and remove the now redundant assignments
    from device class init functions.
    
    Signed-off-by: Markus Armbruster <armbru@redhat.com>
    Reviewed-by: Marcel Apfelbaum <marcel.a@redhat.com>
    Signed-off-by: Andreas Färber <afaerber@suse.de>

Always good to point to specific commits in commit messages instead of
hand-waving "recent changes".

> This brings the capability back and puts SPAPR PHB into "bridge"
> category.

Look, a sysbus device that grabs the resources it needs from its init()
callback instead of getting connected to them by the code that creates
it!  I'm not sure that's proper, but if it works...  Maybe Andreas
(cc'ed) can advise.

> This is not much use for emulated PHB but it is absolutely required
> for VFIO as we put an IOMMU group onto a separate PHB on SPAPR.
>
> Cc: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>
> Are -device and device_add considered synonims? SPAPR PHB can be added
> via the command line just fine but cannot from device_add as
> "Bus 'main-system-bus' does not support hotplugging".

-device is cold plug, device_add is hot plug.  device_add could be
improved to do cold plug when used before the machine starts.