On Mon, 27 Aug 2018 16:02:39 +0200
Greg Kurz <gr...@kaod.org> wrote:
> On Mon, 27 Aug 2018 13:07:10 +0200
> Igor Mammedov <imamm...@redhat.com> wrote:
>
> > The first cpu unplug wasn't ever supported and corresponding
> > monitor/qmp commands refuse to unplug it. However guest is able
> > to issue eject request either using following command:
> > # echo 1 >/sys/devices/system/cpu/cpu0/firmware_node/eject
>
> I can't reproduce the issue with a pc guest and current master...
>
> All I seem to get is an error in dmesg:
>
> [ 97.435446] processor cpu0: Offline failed.
To be able to unplug CPU0 one need to set CONFIG_BOOTPARAM_HOTPLUG_CPU0=y
(I've tested it with a RHLE7 kernel)
> > or directly writing to cpu hotplug registers, which makes
> > qemu crash with SIGSEGV following back trace:
> >
> > kvm_flush_coalesced_mmio_buffer ()
> > while (ring->first != ring->last)
> > ...
> > qemu_flush_coalesced_mmio_buffer
> > prepare_mmio_access
> > flatview_read_continue
> > flatview_read
> > address_space_read_full
> > address_space_rw
> > kvm_cpu_exec(cpu!0)
> > qemu_kvm_cpu_thread_fn
> >
> > the reason for which is that ring == KVMState::coalesced_mmio_ring
> > happens to be a part of 1st CPU that was uplugged by guest.
> >
> > Fix it by forbidding 1st cpu unplug from guest side and in addition
> > remove CPU0._EJ0 ACPI method to make clear that unplug of the first
> > CPU is not supported.
> >
> > Signed-off-by: Igor Mammedov <imamm...@redhat.com>
> > ---
> > CCing spapr and s390x folks in case targets need to prevent 1st CPU unplug
> > as well
> >
>
> A spapr guest can _release_ the first cpu by doing something like:
>
> # echo -n "/cpus/PowerPC,POWER8@0" > /sys/devices/system/cpu/release
>
> But AFAIK, this doesn't unplug the cpu from a QEMU standpoint.
Good,
so only x86 fix should be fixed.
> > CC: m...@redhat.com
> > CC: pbonz...@redhat.com
> > CC: da...@gibson.dropbear.id.au
> > CC: gr...@kaod.org
> > CC: da...@redhat.com
> > CC: coh...@redhat.com
> > ---
> > hw/acpi/cpu.c | 10 ++++++----
> > 1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> > index 5ae595e..4bb8371 100644
> > --- a/hw/acpi/cpu.c
> > +++ b/hw/acpi/cpu.c
> > @@ -117,7 +117,7 @@ static void cpu_hotplug_wr(void *opaque, hwaddr addr,
> > uint64_t data,
> > DeviceState *dev = NULL;
> > HotplugHandler *hotplug_ctrl = NULL;
> >
> > - if (!cdev->cpu) {
> > + if (!cdev->cpu || cdev->cpu == first_cpu) {
> > trace_cpuhp_acpi_ejecting_invalid_cpu(cpu_st->selector);
> > break;
> > }
> > @@ -541,9 +541,11 @@ void build_cpus_aml(Aml *table, MachineState *machine,
> > CPUHotplugFeatures opts,
> > aml_buffer(madt_buf->len, (uint8_t *)madt_buf->data)));
> > g_array_free(madt_buf, true);
> >
> > - method = aml_method("_EJ0", 1, AML_NOTSERIALIZED);
> > - aml_append(method, aml_call1(CPU_EJECT_METHOD, uid));
> > - aml_append(dev, method);
> > + if (CPU(arch_ids->cpus[i].cpu) != first_cpu) {
> > + method = aml_method("_EJ0", 1, AML_NOTSERIALIZED);
> > + aml_append(method, aml_call1(CPU_EJECT_METHOD, uid));
> > + aml_append(dev, method);
> > + }
> >
> > method = aml_method("_OST", 3, AML_SERIALIZED);
> > aml_append(method,
>
