Re: [Qemu-devel] [for-2.11 PATCH 26/26] spapr: add hotplug hooks for PHB hotplug

[Greg Kurz]

On Fri, 28 Jul 2017 14:24:03 +1000
David Gibson <address@hidden> wrote:

> On Thu, Jul 27, 2017 at 07:09:55PM +0200, Greg Kurz wrote:
> > On Thu, 27 Jul 2017 14:41:31 +1000
> > Alexey Kardashevskiy <address@hidden> wrote:
> >   
> > > On 26/07/17 18:40, Greg Kurz wrote:  
> > > > Hotplugging PHBs is a machine-level operation, but PHBs reside on the
> > > > main system bus, so we register spapr machine as the handler for the
> > > > main system bus.
> > > > 
> > > > Signed-off-by: Michael Roth <address@hidden>
> > > > Signed-off-by: Greg Kurz <address@hidden>
> > > > ---
> > > > - rebased against ppc-for-2.10
> > > > - converted to unplug_request
> > > > - handle drc_id at pre-plug
> > > > - reset hotplugged PHB at plug
> > > > - compatibility with older machine types
> > > > ---
> > > >  hw/ppc/spapr.c              |  114 
> > > > +++++++++++++++++++++++++++++++++++++++++++
> > > >  hw/ppc/spapr_drc.c          |    1 
> > > >  hw/ppc/spapr_pci.c          |    2 -
> > > >  include/hw/pci-host/spapr.h |    2 +
> > > >  include/hw/ppc/spapr.h      |    1 
> > > >  5 files changed, 118 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > > index 90485054c2e7..589f76ef9fb8 100644
> > > > --- a/hw/ppc/spapr.c
> > > > +++ b/hw/ppc/spapr.c
> > > > @@ -2540,6 +2540,10 @@ static void ppc_spapr_init(MachineState *machine)
> > > >      register_savevm_live(NULL, "spapr/htab", -1, 1,
> > > >                           &savevm_htab_handlers, spapr);
> > > >  
> > > > +    if (spapr->dr_phb_enabled) {
> > > > +        qbus_set_hotplug_handler(sysbus_get_default(), 
> > > > OBJECT(machine), NULL);
> > > > +    }
> > > > +
> > > >      qemu_register_boot_set(spapr_boot_set, spapr);
> > > >  
> > > >      if (kvm_enabled()) {
> > > > @@ -3238,6 +3242,103 @@ out:
> > > >      error_propagate(errp, local_err);
> > > >  }
> > > >  
> > > > +static void spapr_phb_pre_plug(HotplugHandler *hotplug_dev, 
> > > > DeviceState *dev,
> > > > +                               Error **errp)
> > > > +{
> > > > +    sPAPRPHBState *sphb = SPAPR_PCI_HOST_BRIDGE(dev);
> > > > +
> > > > +    if (sphb->drc_id == (uint32_t)-1) {
> > > > +        sphb->drc_id = sphb->index;
> > > > +    }
> > > > +
> > > > +    if (sphb->drc_id >= SPAPR_DRC_MAX_PHB) {
> > > > +        error_setg(errp, "PHB id %d out of range", sphb->drc_id);
> > > > +    }    
> > > 
> > > 
> > > sphb->index in considered 16bits in the existing code (even though it is
> > > defined as 32bit) and SPAPR_DRC_MAX_PHB is just 256. I'd suggest using the
> > > same limit for both, either 256 or 65536 is fine for me.
> > > 
> > > It is actually a bit weird - it is possible to completely configure few
> > > PHBs in the command line so they will have index==-1 but PCI hotplug code 
> > > -
> > > spapr_phb_get_pci_func_drc() and spapr_phb_realize() - does not check for
> > > this and just does (sphb->index << 16).  
> > 
> > You're right and this looks like a bug... I'll try to come up with a fix.
> >   
> > > May be just ditch drc_id, enforce index not to be -1 and use it as drc_id?
> > >   
> > 
> > This was how Mike did it in the original patchset but David suggested
> > to introduce drc_id (to preserve existing setups I guess):
> > 
> > http://patchwork.ozlabs.org/patch/466262/  
> 
> Huh.  So I did.  But.. sorry, I've changed my mind.
> 
> The fact that needing a DRC forces us to have a reasonable small id
> for each PHB seems like a good excuse to make index mandatory - I'm
> not convinced anyone was actually creating PHBs without index, and
> this does allow us to simplify a bunch of things.
> 
> I'd like to see that done as a preliminary cleanup patch, though.
> 

Just to be sure. I could verify that the weirdness reported by Alexey
causes QEMU to misbehave. Only the first "index-less" PHB has realized
DRCs:

=> subsequent "index-less" PHBs silently ignore hotplugging of PCI devices

=> QEMU won't even start with coldplugged devices in these "index-less"
   PHBs

This preliminary cleanup for hotpluggable PHBs is hence also a bug fix
for current PHBs.

Do we want to fix this long-standing bug in 2.10 ?

Do we want to preserve the current buggy behavior for older machine types ?

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