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

[Greg Kurz]

On Mon, 18 Jan 2021 12:12:03 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> 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.
> 

Yes I agree it's fine if linux enforces some safeguard to prevent
a brutal shutdown when writing 0 to /sys/devices/system/cpu/cpu?/online
within the guest. But in this case, off-lining is part of the usual CPU
unplug sequence, which was requested by the host : I don't think the
safeguard is relevant in this case. This PAPR _feature_ is just another
way of uncleanly shutting down the guest.

> > > 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.
> 

Well... it doesn't finalize the hot-unplug sequence, and we have no
way to cope with that except a machine reset. So I would nearly say
this is working as expected : CPU hot unplug was requested and we
wait for the guest to release the CPU. Linux not wanting to release
the CPU until next reboot for some reason isn't really our concern.

> > > 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.
> > > 

An unplug request can be accepted but its handling can still race with some
manual off-lining in the guest, which would leave us in the very same situation.
So I don't think this patch fixes anything actually (TOCTOU).

I tend to think that mixing manual CPU off-lining and CPU hot-unplug
is probably not the best thing to do in the first place, unless one
really knows what they're doing. Maybe we should rather document the
caveats instead of adding workarounds for what remains a corner case ?

> > > 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;
> > >  }
> > >  
> > 
>

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