Re: [Qemu-devel] [PATCH v1 2/4] s390x/zpci: use hotplug_dev instead of looking up the host bridge

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 7 Nov 2018 15:28:31 -0500
Collin Walling <walling@linux.ibm.com> wrote:

> On 11/5/18 6:50 AM, David Hildenbrand wrote:
> > On 05.11.18 12:40, Christian Borntraeger wrote:  
> >>
> >>
> >> On 11/05/2018 12:37 PM, David Hildenbrand wrote:  
> >>> On 05.11.18 12:21, Cornelia Huck wrote:  
> >>>> On Mon,  5 Nov 2018 12:03:11 +0100
> >>>> David Hildenbrand <david@redhat.com> wrote:
> >>>>  
> >>>>> We directly have it in our hands.
> >>>>>
> >>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
> >>>>> ---
> >>>>>  hw/s390x/s390-pci-bus.c | 4 ++--
> >>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> >>>>> index 1eaae3aca6..68660eac74 100644
> >>>>> --- a/hw/s390x/s390-pci-bus.c
> >>>>> +++ b/hw/s390x/s390-pci-bus.c
> >>>>> @@ -814,9 +814,9 @@ static bool s390_pci_alloc_idx(S390pciState *s, S390PCIBusDevice *pbdev)
> >>>>>  static void s390_pcihost_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >>>>>                                Error **errp)
> >>>>>  {
> >>>>> +    S390pciState *s = S390_PCI_HOST_BRIDGE(hotplug_dev);
> >>>>>      PCIDevice *pdev = NULL;
> >>>>>      S390PCIBusDevice *pbdev = NULL;
> >>>>> -    S390pciState *s = s390_get_phb();
> >>>>>  
> >>>>>      if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_BRIDGE)) {
> >>>>>          BusState *bus;
> >>>>> @@ -924,11 +924,11 @@ static void s390_pcihost_timer_cb(void *opaque)
> >>>>>  static void s390_pcihost_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >>>>>                                  Error **errp)
> >>>>>  {
> >>>>> +    S390pciState *s = S390_PCI_HOST_BRIDGE(hotplug_dev);
> >>>>>      PCIDevice *pci_dev = NULL;
> >>>>>      PCIBus *bus;
> >>>>>      int32_t devfn;
> >>>>>      S390PCIBusDevice *pbdev = NULL;
> >>>>> -    S390pciState *s = s390_get_phb();
> >>>>>  
> >>>>>      if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_BRIDGE)) {
> >>>>>          error_setg(errp, "PCI bridge hot unplug currently not supported");  
> >>>>
> >>>> Not sure whether that is an improvement (s390_get_phb() caches the
> >>>> value, and is called from multiple other places as well.)
> >>>>  
> >>>
> >>> Looking up a variable that is directly passed as an argument doesn't
> >>> look clean to me.  
> >>
> >> I think there was a reason for this caching, namely that qom resolution can
> >> be quite expensive. For the hotplug case this obviously does not matter but
> >> for all the other cases it might. So do we really want to have different 
> >> places use different methods?
> >>  
> > 
> > Caching resolution is fine (as that is expensive), caching a downcast is
> > as far as I remember not necessary. Especially, as you said, for hotplug
> > handlers.

Yes, the complete QOM cast was the expensive thing AFAIR.

> > 
> > Anyhow, if there are strong feelings to this change, I can drop this
> > patch. There are certainly more important things to do in zPCI hotplug code.
> > 
> >   
> 
> Truthfully, I'm not in favor of one over the other. As long as the device handlers
> are consistent, I think either is fine.

I don't feel *that* strong about this change here, either :) Your call.

> 
> However, it would be nice if at some point during plug we cache the PHB somewhere.
> That would be some sort of best-of-both-worlds approach.

Not sure if caching-from-a-downcast would be conceptionally clean. I'd
vote for either taking this patch or dropping it completely.