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 <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. 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 <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; > > } > > > -- 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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