Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu

[David Gibson]

On Mon, Aug 27, 2018 at 04:02:39PM +0200, Greg Kurz wrote:
> On Mon, 27 Aug 2018 13:07:10 +0200
> Igor Mammedov <address@hidden> 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.
> 
> > 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 <address@hidden>
> > ---
> > 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,address@hidden" > /sys/devices/system/cpu/release
> 
> But AFAIK, this doesn't unplug the cpu from a QEMU standpoint.

Unplugging CPU 0 with device_del should be ok too.

-- 
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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