Re: [PATCH] spapr/xive: skip partially initialized vCPUs in presenter

[Greg Kurz <groug@kaod.org>]

On Tue, 1 Oct 2019 13:56:10 +0200
Cédric Le Goater <clg@kaod.org> wrote:

> On 01/10/2019 13:06, Greg Kurz wrote:
> > On Tue,  1 Oct 2019 10:57:22 +0200
> > Cédric Le Goater <clg@kaod.org> wrote:
> > 
> >> When vCPUs are hotplugged, they are added to the QEMU CPU list before
> >> being fully realized. This can crash the XIVE presenter because the
> >> 'tctx' pointer is not necessarily initialized when looking for a
> >> matching target.
> >>
> > 
> > Ouch... :-\
> > 
> >> These vCPUs are not valid targets for the presenter. Skip them.
> >>
> > 
> > This likely fixes this specific issue, but more generally, this
> > seems to indicate that using CPU_FOREACH() is rather fragile.
> > 
> > What about tracking XIVE TM contexts with a QLIST in xive.c ?
> 
> This is a good idea.  
> 
> On HW, the thread interrupt contexts belong to the XIVE presenter 
> subengine. This is the logic doing the CAM line matching to find
> a target for an event notification. So we should model the context 
> list below the XiveRouter in QEMU which models both router and 
> presenter subengines. We have done without a presenter model for 
> the moment and I don't think we will need to introduce one.  
> 
> This would be a nice improvements of my patchset adding support
> for xive escalations and better support of multi chip systems. 
> I have introduced a PNV_CHIP_CPU_FOREACH in this patchset which 
> would become useless with a list of tctx under the XIVE interrupt
> controller, XiveRouter, SpaprXive, PnvXive.
> 

I agree. It makes more sense to have the list below the XiveRouter,
rather than relying on CPU_FOREACH(), which looks a bit weird from
a device emulation code perspective.

> Next step would be to get rid of the tctx->cs pointer. In my latest
> patches, it is only used to calculate the HW CAM line. 
> 
> There might be some consequences on the object hierarchy and it will
> break migration.
> 

This could break if the contexts were devices sitting in a bus, which
isn't the case here. I'll try to come up with a proposal for spapr,
and we can work out the changes on your recent XIVE series for pnv.

> Thanks,
> 
> C.
> 
> > 
> > ================================================================================
> > diff --git a/include/hw/ppc/xive.h b/include/hw/ppc/xive.h
> > index 6d38755f8459..89b9ef7f20b1 100644
> > --- a/include/hw/ppc/xive.h
> > +++ b/include/hw/ppc/xive.h
> > @@ -319,6 +319,8 @@ typedef struct XiveTCTX {
> >      qemu_irq    os_output;
> >  
> >      uint8_t     regs[XIVE_TM_RING_COUNT * XIVE_TM_RING_SIZE];
> > +
> > +    QTAILQ_ENTRY(XiveTCTX) next;
> >  } XiveTCTX;
> >  
> >  /*
> > diff --git a/hw/intc/xive.c b/hw/intc/xive.c
> > index b7417210d817..f7721c711041 100644
> > --- a/hw/intc/xive.c
> > +++ b/hw/intc/xive.c
> > @@ -568,6 +568,8 @@ static void xive_tctx_reset(void *dev)
> >          ipb_to_pipr(tctx->regs[TM_QW3_HV_PHYS + TM_IPB]);
> >  }
> >  
> > +static QTAILQ_HEAD(, XiveTCTX) xive_tctx_list = QTAILQ_HEAD_INITIALIZER(xive_tctx_list);
> > +
> >  static void xive_tctx_realize(DeviceState *dev, Error **errp)
> >  {
> >      XiveTCTX *tctx = XIVE_TCTX(dev);
> > @@ -609,10 +611,14 @@ static void xive_tctx_realize(DeviceState *dev, Error **errp)
> >      }
> >  
> >      qemu_register_reset(xive_tctx_reset, dev);
> > +    QTAILQ_INSERT_HEAD(&xive_tctx_list, tctx, next);
> >  }
> >  
> >  static void xive_tctx_unrealize(DeviceState *dev, Error **errp)
> >  {
> > +    XiveTCTX *tctx = XIVE_TCTX(dev);
> > +
> > +    QTAILQ_REMOVE(&xive_tctx_list, tctx, next);
> >      qemu_unregister_reset(xive_tctx_reset, dev);
> >  }
> >  
> > @@ -1385,15 +1391,14 @@ static bool xive_presenter_match(XiveRouter *xrtr, uint8_t format,
> >                                   bool cam_ignore, uint8_t priority,
> >                                   uint32_t logic_serv, XiveTCTXMatch *match)
> >  {
> > -    CPUState *cs;
> > +    XiveTCTX *tctx;
> >  
> >      /*
> >       * TODO (PowerNV): handle chip_id overwrite of block field for
> >       * hardwired CAM compares
> >       */
> >  
> > -    CPU_FOREACH(cs) {
> > -        XiveTCTX *tctx = xive_router_get_tctx(xrtr, cs);
> > +    QTAILQ_FOREACH(tctx, &xive_tctx_list, next) {
> >          int ring;
> >  
> >          /*
> > ================================================================================
> > 
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >> ---
> >>  hw/intc/xive.c | 8 ++++++++
> >>  1 file changed, 8 insertions(+)
> >>
> >> diff --git a/hw/intc/xive.c b/hw/intc/xive.c
> >> index b7417210d817..29df06df1136 100644
> >> --- a/hw/intc/xive.c
> >> +++ b/hw/intc/xive.c
> >> @@ -1396,6 +1396,14 @@ static bool xive_presenter_match(XiveRouter *xrtr, uint8_t format,
> >>          XiveTCTX *tctx = xive_router_get_tctx(xrtr, cs);
> >>          int ring;
> >>  
> >> +        /*
> >> +         * Skip partially initialized vCPUs. This can happen when
> >> +         * vCPUs are hotplugged.
> >> +         */
> >> +        if (!tctx) {
> >> +            continue;
> >> +        }
> >> +
> >>          /*
> >>           * HW checks that the CPU is enabled in the Physical Thread
> >>           * Enable Register (PTER).
> > 
>