On Thu, 14 Jan 2021 15:06:28 -0300 Daniel Henrique Barboza <danielhb...@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 <x...@redhat.com> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1911414 > Signed-off-by: Daniel Henrique Barboza <danielhb...@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; > } >
