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Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug t
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu |
Date: |
Wed, 29 Aug 2018 17:23:34 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 |
On 08/29/18 15:51, Igor Mammedov wrote:
> On Wed, 29 Aug 2018 09:15:53 -0400
> "Michael S. Tsirkin" <address@hidden> wrote:
>
>> On Wed, Aug 29, 2018 at 10:43:11AM +0200, Igor Mammedov wrote:
>>> On Wed, 29 Aug 2018 12:54:40 +1000
>>> David Gibson <address@hidden> wrote:
>>>
>>>> On Tue, Aug 28, 2018 at 03:18:48PM +0200, Igor Mammedov wrote:
>>>>> On Tue, 28 Aug 2018 10:52:37 +1000
>>>>> David Gibson <address@hidden> 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 <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.
>>>>> Do you mean real unplugging (cpu0 object freed) or just remove request?
>>>>>
>>>>
>>>> Real unplugging should be possible. I'm not sure how thorougly it's
>>>> been tested, though.
>>> Well, common kvm code in qemu seems to be in disagreement with it
>>> as backtrace in this patch shows also usage of first_cpu macro
>>> won't survive such unplug.
>>
>> Paolo - any take on this? Do we need to make cpu 0 special like this?
> It probably would take a bunch of refactoring to get rid of first_cpu&co
> dependencies and besides of experimenting with cpu0 unplug in guest kernel
> there isn't any other value in it, so it probably not worth the effort.
>
> On top of that, for pc/q35 machine we would need to select boot cpu
> in some other way (right now it's hardwired to first_cpu).
>
> It seems that seabios might work if cpu0 isn't present, don't know about OVMF.
>
Sorry, I have no idea. I'm not aware of any OVMF testing with partially
populated VCPU topologies.
Thanks
Laszlo
- [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Igor Mammedov, 2018/08/27
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, David Hildenbrand, 2018/08/27
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Greg Kurz, 2018/08/27
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Igor Mammedov, 2018/08/27
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, David Gibson, 2018/08/27
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Igor Mammedov, 2018/08/28
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, David Gibson, 2018/08/29
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Igor Mammedov, 2018/08/29
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Michael S. Tsirkin, 2018/08/29
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu, Igor Mammedov, 2018/08/29
- Re: [Qemu-devel] [PATCH] pc: make sure that guest isn't able to unplug the first cpu,
Laszlo Ersek <=