Re: [Qemu-devel] [PATCH] spapr: fix memory hotplug error path

[Greg Kurz <groug@kaod.org>]

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On Tue, 4 Jul 2017 09:20:50 +0530
Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:

> On Tue, Jul 04, 2017 at 09:01:43AM +0530, Bharata B Rao wrote:
> > On Mon, Jul 03, 2017 at 02:21:31PM +0200, Greg Kurz wrote:  
> > > QEMU shouldn't abort if spapr_add_lmbs()->spapr_drc_attach() fails.
> > > Let's propagate the error instead, like it is done everywhere else
> > > where spapr_drc_attach() is called.
> > > 
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  hw/ppc/spapr.c |   10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index 70b3fd374e2b..e103be500189 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -2601,6 +2601,7 @@ static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
> > >      int i, fdt_offset, fdt_size;
> > >      void *fdt;
> > >      uint64_t addr = addr_start;
> > > +    Error *local_err = NULL;
> > > 
> > >      for (i = 0; i < nr_lmbs; i++) {
> > >          drc = spapr_drc_by_id(TYPE_SPAPR_DRC_LMB,
> > > @@ -2611,7 +2612,12 @@ static void spapr_add_lmbs(DeviceState *dev, uint64_t addr_start, uint64_t size,
> > >          fdt_offset = spapr_populate_memory_node(fdt, node, addr,
> > >                                                  SPAPR_MEMORY_BLOCK_SIZE);
> > > 
> > > -        spapr_drc_attach(drc, dev, fdt, fdt_offset, errp);
> > > +        spapr_drc_attach(drc, dev, fdt, fdt_offset, &local_err);
> > > +        if (local_err) {
> > > +            g_free(fdt);
> > > +            error_propagate(errp, local_err);
> > > +            return;
> > > +        }  
> > 
> > There is some history to this. I was doing error recovery and propagation
> > here similarly during memory hotplug development phase until Igor
> > suggested that we shoudn't try to recover after we have done guest
> > visible changes.
> > 
> > Refer to "changes in v6" section in this post:
> > https://lists.gnu.org/archive/html/qemu-ppc/2015-06/msg00296.html
> > 
> > However at that time we were doing memory add by DRC index method
> > and hence would attach and online one LMB at a time.
> > In that method, if an intermediate attach fails we would end up with a few
> > LMBs being onlined by the guest already. However subsequently
> > we have switched (optionally, based on dedicated_hp_event_source) to
> > count-indexed method of hotplug where we do attach of all LMBs one by one
> > and then request the guest to hotplug all of them at once using count-indexed
> > method.
> > 
> > So it will be a bit tricky to abort for index based case and recover
> > correctly for count-indexed case.  
> 
> Looked at the code again and realized that though we started with
> index based LMB addition, we later switched to count based addition. Then
> we added support for count-indexed type subject to the presence
> of dedidated hotplug event source while still retaining the support
> for count based addition.
> 
> So presently we do attach of all LMBs one by one and then do onlining
> (count based or count-indexed based) once. Hence error recovery
> for both cases would be similar now. So I guess you should take care of
> undoing pc_dimm_memory_plug() like Igor mentioned and also undo the
> effects of partial successful attaches.
> 

I've sent a v2 that adds rollback.

Cheers,

--
Greg


> > 
> > Regards,
> > Bharata.  
> 


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