Re: About releasing vcpu when closing vcpu fd

Anshul Makkar
2014-07-11T09:59:48Z

Hi Gu,

Sorry, just wanted to check whether you are going to release patchset
or it will take some more time.

Thanks
Anshul Makkar

On Tue, Jul 1, 2014 at 4:07 AM, Gu Zheng <guz.fnst@cn.fujitsu.com> wrote:
> Hi Anshul,
> On 06/30/2014 10:41 PM, Anshul Makkar wrote:
>
>> Hi,
>>
>> Currently as per the specs for cpu_hot(un)plug, ACPI GPE Block: IO
>> ports 0xafe0-0xafe3 where each bit corresponds to each CPU.
>>
>> Currently, EJ0 method in acpi-dsdt-cpu-hotplu.dsl doesn't do anything.
>>
>> Method(CPEJ, 2, NotSerialized) {
>> // _EJ0 method - eject callback
>> Sleep(200)
>> }
>>
>> I want to implement a notification mechanism for CPU hotunplug just
>> like we have in memory hotunplug where in we write to particular IO
>> port and this read/write is caught in the memory-hotplug.c .
>>
>> So, just want a suggestion as to whether I should expand the IO port
>> range from 0xafe0 to 0xafe4 (addition of 1 byte), where last byte is
>> for notification of EJ0 event.
>>
>> Or if you have any other suggestion, please share.
>
> In fact, Chen Fan has implemented this feature in his previous vcup hot remove
> patchset:
> http://lists.gnu.org/archive/html/qemu-devel/2013-12/msg04266.html
> As you know, it is based on the cleaning up kvm vcpus as you mentioned the in
> previous thread, and it has not been applied for some reason.
> So I am trying to respin a new one based on Chen Fan's previous patchset recently,
> and if nothing else, I will send it to the community in the coming week. So if you
> like, please hold on for a moment.;)
>
> Thanks,
> Gu
>
>>
>> Thanks
>> Anshul Makkar
>>
>> On Fri, Jun 6, 2014 at 3:41 PM, Anshul Makkar
>> <anshul.makkar@profitbricks.com> wrote:
>>> Oh yes, sorry for the ambiguity. I meant proposal to "park" unplugged vcpus.
>>>
>>> Thanks for the suggesting the practical approach.
>>>
>>> Anshul Makkar
>>>
>>> On Fri, Jun 6, 2014 at 3:36 PM, Gleb Natapov <gleb@minantech.com> wrote:
>>>> On Fri, Jun 06, 2014 at 03:02:59PM +0200, Anshul Makkar wrote:
>>>>> IIRC, Igor was of the opinion that patch for vcpu deletion will be
>>>>> incomplete till its handled properly in kvm i.e vcpus are destroyed
>>>>> completely. http://comments.gmane.org/gmane.comp.emulators.kvm.devel/114347
>>>>> .
>>>>>
>>>>> So can the above proposal where just vcpus can be disabled and
>>>>> reused in qemu is an acceptable solution ?
>>>>>
>>>> If by "above proposal" you mean the proposal in the email you linked,
>>>> then no since it tries to destroy vcpu, but does it incorrectly. If you
>>>> mean proposal to "park" unplugged vcpu, so that guest will not be able
>>>> to use it, then yes, it is pragmatic path forward.
>>>>
>>>>
>>>>> Thanks
>>>>> Anshul Makkar
>>>>>
>>>>> On Thu, May 29, 2014 at 10:12 AM, Gleb Natapov <gleb@kernel.org> wrote:
>>>>>> On Thu, May 29, 2014 at 01:40:08PM +0800, Gu Zheng wrote:
>>>>>>>>> There was a patch(from Chen Fan, last august) about releasing vcpu when
>>>>>>>>> closing vcpu fd <http://www.spinics.net/lists/kvm/msg95701.html>, but
>>>>>>>>> your comment said "Attempt where made to make it possible to destroy
>>>>>>>>> individual vcpus separately from destroying VM before, but they were
>>>>>>>>> unsuccessful thus far."
>>>>>>>>> So what is the pain here? If we want to achieve the goal, what should we do?
>>>>>>>>> Looking forward to your further comments.:)
>>>>>>>>>
>>>>>>>> CPU array is accessed locklessly in a lot of places, so it will have to be RCUified.
>>>>>>>> There was attempt to do so 2 year or so ago, but it didn't go anyware. Adding locks is
>>>>>>>> to big a price to pay for ability to free a little bit of memory by destroying vcpu.
>>>>>>>
>>>>>>> Yes, it's a pain here. But if we want to implement "vcpu hot-remove", this must be
>>>>>>> fixed sooner or later.
>>>>>> Why? "vcpu hot-remove" already works (or at least worked in the past
>>>>>> for some value of "work"). No need to destroy vcpu completely, just
>>>>>> park it and tell a guest not to use it via ACPI hot unplug event.
>>>>>>
>>>>>>> And any guys working on kvm "vcpu hot-remove" now?
>>>>>>>
>>>>>>>> An
>>>>>>>> alternative may be to make sure that stopped vcpu takes as little memory as possible.
>>>>>>>
>>>>>>> Yeah. But if we add a new vcpu with the old id that we stopped before, it will fail.
>>>>>>>
>>>>>> No need to create vcpu again, just unpark it and notify a guest via ACPI hot plug event that
>>>>>> vcpu can be used now.
>>>>>>
>>>>>> --
>>>>>> Gleb.
>>>>>> --
>>>>>> To unsubscribe from this list: send the line "unsubscribe kvm" in
>>>>>> the body of a message to majordomo@vger.kernel.org
>>>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>>
>>>> --
>>>> Gleb.
>> .
>>
>
>