Re: [Qemu-devel] [FIX PATCH] spapr_drc: Return correct state for logical DR in entity_sense()

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting David Gibson (2015-09-07 20:22:50)
> On Mon, Sep 07, 2015 at 11:37:04AM +0530, Bharata B Rao wrote:
> > When drmgr is run in the guest to add a device for which device_add
> > hasn't been issued in QEMU, configure-connector call fails.
> > When configure-connector call fails, the guest would release (*)
> > the previously acquired DRC by setting back the DRC isolation state
> > to ISOLATED and allocation state to UNUSABLE. These calls will be issued
> > only if get-sensor-state call returns PRESENT state. However currently for
> > a logical DR, entity_sense() would unconditinally return UNUSABLE
> > state only. This prevents any subsequent hotplug of the device with
> > that DRC.

This seems a little odd. I think we default to ALLOCATION_STATE_UNUSABLE
for logical DR, and it's up the guest to transition to USABLE, which
probably happens prior to the configure-connector calls. So I think the
net effect of this fix is that guest will see these unallocated/unattached
resources the same way they would a resource that was actually attached
via device_add, and all we're really doing is working around the
eventual configuration failure that that will lead to by pretending a
resource was actually there.

According to PAPR+ 2.7:

13.7.3.1 Acquire Logical Resource from Resource Pool:

  If the state is “unusable” the OS issues set-indicator (allocation-state, usable) to attempt to allocate the re-
  source. Similarly, if the state is “available for exchange” the OS issues set-indicator (allocation-state, ex-
  change) to attempt to allocate the resource, and if the state is “available for recovery” the OS issues
  set-indicator (allocation-state, recover) to attempt to allocate the resource.

and

13.7 Logical Resource Dynamic Reconfiguration (LRDR):

  The OS may use the get-sensor-state RTAS call with the dr-entity-sense token to deter-
mine if a given drc-index refers to a connector that is currently usable for DR operations. If the connector is not
currently usable the return state is “DR entity unusable” (2). A set-indicator (isolation state) RTAS call to an unusable
connector or (dr-indicator) to any logical resource connector results in a “No such indicator implemented” return sta-
tus.  

So I think maybe the proper fix is to make sure that
drc->set_indicator_state() fails with an error that indicates to RTAS to
return NO_SENSOR (-3) for cases where we haven't attached a resource
to the DRC via device_add.

Which also kind of re-opens the discussion of whether or not
drc->set_indicator_state() should return RTAS errors directly. I'd
still stray away from that for now but maybe if we get more cases
like this it'll start becoming more practical.

> > 
> > Fix this by returning the right state in entity_sense() by checking
> > the allocation_state of DRC.
> > 
> > (*) https://lists.ozlabs.org/pipermail/linuxppc-dev/2015-September/133430.html
> > 
> > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> and applied to my tree.
> 
> > ---
> >  hw/ppc/spapr_drc.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> > index 9ce844a..2586065 100644
> > --- a/hw/ppc/spapr_drc.c
> > +++ b/hw/ppc/spapr_drc.c
> > @@ -186,7 +186,11 @@ static sPAPRDREntitySense entity_sense(sPAPRDRConnector *drc)
> >               */
> >              state = SPAPR_DR_ENTITY_SENSE_EMPTY;
> >          } else {
> > -            state = SPAPR_DR_ENTITY_SENSE_UNUSABLE;
> > +            if (drc->allocation_state == SPAPR_DR_ALLOCATION_STATE_UNUSABLE) {
> > +                state = SPAPR_DR_ENTITY_SENSE_UNUSABLE;
> > +            } else {
> > +                state = SPAPR_DR_ENTITY_SENSE_PRESENT;
> > +            }
> >          }
> >      }
> >  
> 
> -- 
> 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