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

[Greg Kurz]

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.

> 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.

> 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;
>  }
>