Re: [PATCH 07/23] KVM: PPC: Book3S: Allow reuse of vCPU object

[Alexander Graf <agraf@suse.de>]

On 20.03.15 12:26, Paul Mackerras wrote:
> On Fri, Mar 20, 2015 at 12:01:32PM +0100, Alexander Graf wrote:
>>
>>
>> On 20.03.15 10:39, Paul Mackerras wrote:
>>> From: Bharata B Rao <bharata@linux.vnet.ibm.com>
>>>
>>> Since KVM isn't equipped to handle closure of vcpu fd from userspace(QEMU)
>>> correctly, certain work arounds have to be employed to allow reuse of
>>> vcpu array slot in KVM during cpu hot plug/unplug from guest. One such
>>> proposed workaround is to park the vcpu fd in userspace during cpu unplug
>>> and reuse it later during next hotplug.
>>>
>>> More details can be found here:
>>> KVM: https://www.mail-archive.com/kvm@vger.kernel.org/msg102839.html
>>> QEMU: http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg00859.html
>>>
>>> In order to support this workaround with PowerPC KVM, don't create or
>>> initialize ICP if the vCPU is found to be already associated with an ICP.
>>>
>>> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
>>> Signed-off-by: Paul Mackerras <paulus@samba.org>
>>
>> This probably makes some sense, but please make sure that user space has
>> some way to figure out whether hotplug works at all.
> 
> Bharata is working on the qemu side of all this, so I assume he has
> that covered.

Well, so far the kernel doesn't expose anything he can query, so I
suppose he just blindly assumes that older host kernels will randomly
break and nobody cares. I'd rather prefer to see a CAP exposed that qemu
can check on.

> 
>> Also Paul, for patches that you pick up from others, I'd prefer if they
>> send the patches to the ML themselves first and you pick them up from
>> there then. That way we give everyone the same treatment.
> 
> Fair enough.  In fact Bharata did post the patch but he sent it to
> linuxppc-dev@ozlabs.org not the KVM lists.

Please make sure you only take patches into your queue that made it to
at least kvm@vger, preferably kvm-ppc@vger as well. If you see related
patches on other mailing lists, just ask the respective people to resend
with proper ML exposure.


Alex