Re: [PATCH v1 7/7] spapr.c: consider CPU core online state before allowing unplug

[David Gibson]

On Fri, Jan 15, 2021 at 06:22:16PM +0100, Greg Kurz wrote:
> On Thu, 14 Jan 2021 15:06:28 -0300
> Daniel Henrique Barboza <danielhb413@gmail.com> wrote:
> 
> > The only restriction we have when unplugging CPUs is to forbid unplug of
> > the boot cpu core. spapr_core_unplug_possible() does not contemplate the
> 
> I can't remember why this restriction was introduced in the first place...
> This should be investigated and documented if the limitation still stands.
> 
> > possibility of some cores being offlined by the guest, meaning that we're
> > rolling the dice regarding on whether we're unplugging the last online
> > CPU core the guest has.
> > 
> 
> Trying to unplug the last CPU is obviously something that deserves
> special care. LoPAPR is quite explicit on the outcome : this should
> terminate the partition.
> 
> 13.7.4.1.1. Isolation of CPUs
> 
> The isolation of a CPU, in all cases, is preceded by the stop-self
> RTAS function for all processor threads, and the OS insures that all
> the CPU’s threads are in the RTAS stopped state prior to isolating the
> CPU. Isolation of a processor that is not stopped produces unpredictable
> results. The stopping of the last processor thread of a LPAR partition
> effectively kills the partition, and at that point, ownership of all
> partition resources reverts to the platform firmware.
> 
> R1-13.7.4.1.1-1. For the LRDR option: Prior to issuing the RTAS
> set-indicator specifying isolate isolation-state of a CPU DR
> connector type, all the CPU threads must be in the RTAS stopped
> state.
> 
> R1-13.7.4.1.1-2. For the LRDR option: Stopping of the last processor
> thread of a LPAR partition with the stop-self RTAS function, must kill
> the partition, with ownership of all partition resources reverting to
> the platform firmware.
> 
> This is clearly not how things work today : linux doesn't call
> "stop-self" on the last vCPU and even if it did, QEMU doesn't
> terminate the VM.

> If there's a valid reason to not implement this PAPR behavior, I'd like
> it to be documented.

So, we should implement this in QEMU - if you stop-self the last
thread, it should be the equivalent of a power off.  Linux not ever
doing that probably makes sense - it wants you to encourage you to
shut down properly for data safety.

> > If we hit the jackpot, we're going to detach the core DRC and pulse the
> > hotplug IRQ, but the guest OS will refuse to release the CPU. Our
> > spapr_core_unplug() DRC release callback will never be called and the CPU
> > core object will keep existing in QEMU. No error message will be sent
> > to the user, but the CPU core wasn't unplugged from the guest.
> > 
> > If the guest OS onlines the CPU core again we won't be able to hotunplug it
> > either. 'dmesg' inside the guest will report a failed attempt to offline an
> > unknown CPU:
> > 
> > [  923.003994] pseries-hotplug-cpu: Failed to offline CPU <NULL>, rc: -16
> > 
> > This is the result of stopping the DRC state transition in the middle in the
> > first failed attempt.
> > 
> 
> Yes, at this point only a machine reset can fix things up.
> 
> Given this is linux's choice not to call "stop-self" as it should do, I'm not
> super fan of hardcoding this logic in QEMU, unless there are really good
> reasons to do so.

Uh.. sorry I don't follow how linux is doing something wrong here.

> > We can avoid this, and potentially other bad things from happening, if we
> > avoid to attempt the unplug altogether in this scenario. Let's check for
> > the online/offline state of the CPU cores in the guest before allowing
> > the hotunplug, and forbid removing a CPU core if it's the last one online
> > in the guest.
> > 
> > Reported-by: Xujun Ma <xuma@redhat.com>
> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1911414
> > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > ---
> >  hw/ppc/spapr.c | 39 ++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 38 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index a2f01c21aa..d269dcd102 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -3709,9 +3709,16 @@ static void spapr_core_unplug(HotplugHandler 
> > *hotplug_dev, DeviceState *dev)
> >  static int spapr_core_unplug_possible(HotplugHandler *hotplug_dev, CPUCore 
> > *cc,
> >                                        Error **errp)
> >  {
> > +    CPUArchId *core_slot;
> > +    SpaprCpuCore *core;
> > +    PowerPCCPU *cpu;
> > +    CPUState *cs;
> > +    bool last_cpu_online = true;
> >      int index;
> >  
> > -    if (!spapr_find_cpu_slot(MACHINE(hotplug_dev), cc->core_id, &index)) {
> > +    core_slot = spapr_find_cpu_slot(MACHINE(hotplug_dev), cc->core_id,
> > +                                    &index);
> > +    if (!core_slot) {
> >          error_setg(errp, "Unable to find CPU core with core-id: %d",
> >                     cc->core_id);
> >          return -1;
> > @@ -3722,6 +3729,36 @@ static int spapr_core_unplug_possible(HotplugHandler 
> > *hotplug_dev, CPUCore *cc,
> >          return -1;
> >      }
> >  
> > +    /* Allow for any non-boot CPU core to be unplugged if already offline 
> > */
> > +    core = SPAPR_CPU_CORE(core_slot->cpu);
> > +    cs = CPU(core->threads[0]);
> > +    if (cs->halted) {
> > +        return 0;
> > +    }
> > +
> > +    /*
> > +     * Do not allow core unplug if it's the last core online.
> > +     */
> > +    cpu = POWERPC_CPU(cs);
> > +    CPU_FOREACH(cs) {
> > +        PowerPCCPU *c = POWERPC_CPU(cs);
> > +
> > +        if (c == cpu) {
> > +            continue;
> > +        }
> > +
> > +        if (!cs->halted) {
> > +            last_cpu_online = false;
> > +            break;
> > +        }
> > +    }
> > +
> > +    if (last_cpu_online) {
> > +        error_setg(errp, "Unable to unplug CPU core with core-id %d: it is 
> > "
> > +                   "the only CPU core online in the guest", cc->core_id);
> > +        return -1;
> > +    }
> > +
> >      return 0;
> >  }
> >  
> 

-- 
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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