Re: About releasing vcpu when closing vcpu fd

Anshul Makkar
2014-06-06T13:41:58Z

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.