Re: [Qemu-ppc] [Qemu-devel] [PATCH 2/5] hw/ppc: removing spapr_drc_detach_cb opaques

[David Gibson]

On Wed, May 03, 2017 at 04:33:56PM +0200, Laurent Vivier wrote:
> On 30/04/2017 19:25, Daniel Henrique Barboza wrote:
> > Following up the previous detach_cb change, this patch removes the
> > detach_cb_opaque entirely from the code.
> > 
> > The reason is that the drc->detach_cb_opaque object can't be
> > restored in the post load of the upcoming DRC migration and no detach
> > callbacks actually need this opaque. 'spapr_core_release' is
> > receiving it as NULL, 'spapr_phb_remove_pci_device_cb' is receiving
> > a phb object as opaque but is't using it. These were trivial removal
> > cases.
> > 
> > However, the LM removal callback 'spapr_lmb_release' is receiving
> > and using the opaque object, a 'sPAPRDIMMState' struct. This struct
> > holds the number of LMBs the DIMM object contains and the callback
> > was using this counter as a countdown to check if all LMB DRCs were
> > release before proceeding to the DIMM unplug. To remove the need of
> > this callback we have choices such as:
> > 
> > - migrate the 'sPAPRDIMMState' struct. This would require creating a
> > QTAILQ to store all DIMMStates and an additional 'dimm_id' field to
> > associate the DIMMState with the DIMM object. We could attach this
> > QTAILQ to the 'sPAPRPHBState' and retrieve it later in the callback.
> > 
> 
> Did you think about adding the nr_lmbs into PCDIMMDevice structure?

I think that's unlikely to fly, because it's very platform specific
state to put into the supposedly general DIMM object.

> Or to create a SPAPRPCDIMMDevice with PCDIMMDevice parent_obj and
> nr_lmbs field we can retrieve from the DeviceState pointer?

I wondered about that.  In some ways it is the simplest way forward;
however it means the user (and/or libvirt) has to be aware that they
need an spapr dimm not a pc-dimm for this platform, which is pretty
awful UX.

Plus, if we were going to have an SPAPR specific way of handling
memory hotplug, I'd prefer to really base it on sPAPR, which would let
us get rid of a bunch of the ugly glue between DIMMs and LMBs.

> 
> I don't know if it is a good idea or an acceptable way to do that, but
> I'd like to know if you have thought about that.
> 
> Thanks,
> Laurent
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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