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

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

Quoting Michael Roth (2015-09-08 16:03:56)
> 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.

Patch incoming:

  spapr_drc: don't allow 'empty' DRCs to be unisolated

applies to spapr-next but requires revert of this patch.

Bharata, can you give it a spin with CPU hotplug and see if it fixes the
issue you hit?

> 
> 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.

I'm starting to second-guess myself on this. I'm trying to maintain
separation between RTAS/DRC but result is a bit pathological. Feel free
to comment in the above patch.

Just FYI: original author names appear to have gotten lost in recent
spapr-next rebase.

> 
> > > 
> > > 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
> 
>