Re: About releasing vcpu when closing vcpu fd

[Anshul Makkar <anshul.makkar@profitbricks.com>]

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.
>> .
>>
>
>