From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59101) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fudtd-0001EL-5k for qemu-devel@nongnu.org; Tue, 28 Aug 2018 09:19:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fudtZ-0000ar-5d for qemu-devel@nongnu.org; Tue, 28 Aug 2018 09:19:01 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:45432 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fudtZ-0000aO-08 for qemu-devel@nongnu.org; Tue, 28 Aug 2018 09:18:57 -0400 Date: Tue, 28 Aug 2018 15:18:48 +0200 From: Igor Mammedov Message-ID: <20180828151848.3d09ca06@redhat.com> In-Reply-To: <20180828005237.GC2222@umbus.fritz.box> References: <1535368030-214556-1-git-send-email-imammedo@redhat.com> <20180827160239.54cc993a@bahia.lan> <20180828005237.GC2222@umbus.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: Greg Kurz , qemu-devel@nongnu.org, mst@redhat.com, pbonzini@redhat.com, david@redhat.com, cohuck@redhat.com On Tue, 28 Aug 2018 10:52:37 +1000 David Gibson wrote: > On Mon, Aug 27, 2018 at 04:02:39PM +0200, Greg Kurz wrote: > > On Mon, 27 Aug 2018 13:07:10 +0200 > > Igor Mammedov 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 > > > --- > > > 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. > > Unplugging CPU 0 with device_del should be ok too. Do you mean real unplugging (cpu0 object freed) or just remove request?