Re: [PATCH v2 4/4] ppc/spapr: Don't kill the guest if a recovered FWNMI machine check delivery fails

[David Gibson]

On Wed, Mar 25, 2020 at 07:13:32PM +0100, Greg Kurz wrote:
> On Thu, 26 Mar 2020 00:29:06 +1000
> Nicholas Piggin <address@hidden> wrote:
> 
> > Try to be tolerant of FWNMI delivery errors if the machine check had been
> > recovered by the host.
> > 
> > Signed-off-by: Nicholas Piggin <address@hidden>
> > ---
> >  hw/ppc/spapr_events.c | 27 ++++++++++++++++++++++-----
> >  1 file changed, 22 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c
> > index c8964eb25d..b90ecb8afe 100644
> > --- a/hw/ppc/spapr_events.c
> > +++ b/hw/ppc/spapr_events.c
> > @@ -833,13 +833,25 @@ static void spapr_mce_dispatch_elog(PowerPCCPU *cpu, 
> > bool recovered)
> >      /* get rtas addr from fdt */
> >      rtas_addr = spapr_get_rtas_addr();
> >      if (!rtas_addr) {
> > -        error_report(
> > +        if (!recovered) {
> > +            error_report(
> >  "FWNMI: Unable to deliver machine check to guest: rtas_addr not found.");
> > -        qemu_system_guest_panicked(NULL);
> > +            qemu_system_guest_panicked(NULL);
> > +        } else {
> > +            warn_report(
> > +"FWNMI: Unable to deliver machine check to guest: rtas_addr not found. "
> > +"Machine check recovered.");
> > +        }
> >          g_free(ext_elog);
> >          return;
> >      }
> >  
> > +    /*
> > +     * Must not set interlock if the MCE does not get delivered to the 
> > guest
> > +     * in the error case above.
> > +     */
> 
> It is a bit confusing to read "must not set interlock" and to see the
> interlock being set the line below IM-non-native-speaker-HO... also
> a small clarification of the outcome of taking the interlock without
> delivering the MCE could help people who aren't familiar with FWNMI
> to avoid doing bad things.

That's a good point.  That comment made sense to me (and presumably to
Nick) because it directly answers a question I had on an earlier draft
(which moved this without comment).  But someone without that context
(say, future me) could well find that confusing.

> 
> What about something like the following ?
> 
>     /*
>      * By taking the interlock, we assume that the MCE will be
>      * delivered to the guest. CAUTION: don't add anything that
>      * could prevent the MCE to be delivered after this line,
>      * otherwise the guest won't be able to release the interlock
>      * and ultimately hang/crash?
>      */

I've substituted your comment inline.

> 
> > +    spapr->fwnmi_machine_check_interlock = cpu->vcpu_id;
> > +
> 
> For improved paranoia, this could even be done just before calling
> ppc_cpu_do_fwnmi_machine_check().
> 
> Anyway, the change is good enough so:
> 
> Reviewed-by: Greg Kurz <address@hidden>
> 
> >      stq_be_phys(&address_space_memory, rtas_addr + RTAS_ERROR_LOG_OFFSET,
> >                  env->gpr[3]);
> >      cpu_physical_memory_write(rtas_addr + RTAS_ERROR_LOG_OFFSET +
> > @@ -876,9 +888,15 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool 
> > recovered)
> >           * that CPU called "ibm,nmi-interlock")
> >           */
> >          if (spapr->fwnmi_machine_check_interlock == cpu->vcpu_id) {
> > -            error_report(
> > +            if (!recovered) {
> > +                error_report(
> >  "FWNMI: Unable to deliver machine check to guest: nested machine check.");
> > -            qemu_system_guest_panicked(NULL);
> > +                qemu_system_guest_panicked(NULL);
> > +            } else {
> > +                warn_report(
> > +"FWNMI: Unable to deliver machine check to guest: nested machine check. "
> > +"Machine check recovered.");
> > +            }
> >              return;
> >          }
> >          
> > qemu_cond_wait_iothread(&spapr->fwnmi_machine_check_interlock_cond);
> > @@ -906,7 +924,6 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool 
> > recovered)
> >          warn_report("Received a fwnmi while migration was in progress");
> >      }
> >  
> > -    spapr->fwnmi_machine_check_interlock = cpu->vcpu_id;
> >      spapr_mce_dispatch_elog(cpu, recovered);
> >  }
> >  
> 

-- 
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

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