From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:52105)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <guz.fnst@cn.fujitsu.com>) id 1Xxvo1-0006UH-EN
	for qemu-devel@nongnu.org; Mon, 08 Dec 2014 05:44:47 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <guz.fnst@cn.fujitsu.com>) id 1Xxvnv-0003uv-8e
	for qemu-devel@nongnu.org; Mon, 08 Dec 2014 05:44:41 -0500
Received: from [59.151.112.132] (port=31602 helo=heian.cn.fujitsu.com)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <guz.fnst@cn.fujitsu.com>) id 1Xxvnu-0003uo-Sa
	for qemu-devel@nongnu.org; Mon, 08 Dec 2014 05:44:35 -0500
Message-ID: <54857D61.4030502@cn.fujitsu.com>
Date: Mon, 8 Dec 2014 18:28:49 +0800
From: Gu Zheng <guz.fnst@cn.fujitsu.com>
MIME-Version: 1.0
References: <1409197002-9498-1-git-send-email-guz.fnst@cn.fujitsu.com>
	<1409197002-9498-11-git-send-email-guz.fnst@cn.fujitsu.com>
	<CAGZKiBqnNPS0Cdj+kSpJp9TzW6n1O-U+=7RERsTKnYeesY8UOw@mail.gmail.com>
	<CAFEAcA-Pkf-cpa+rucuVxfLL-MfiancGTtygHDvcXMJ-Ee7fPg@mail.gmail.com>
In-Reply-To: <CAFEAcA-Pkf-cpa+rucuVxfLL-MfiancGTtygHDvcXMJ-Ee7fPg@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [RFC V2 10/10] cpus: reclaim allocated vCPU objects
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, tangchen@cn.fujitsu.com, isimatu.yasuaki@jp.fujitsu.com, Bharata B Rao <bharata.rao@gmail.com>, ChenFan <chen.fan.fnst@cn.fujitsu.com>, Anshul Makkar <anshul.makkar@profitbricks.com>, "afaerber@suse.de" <afaerber@suse.de>

Hi Peter,
On 12/08/2014 05:26 PM, Peter Maydell wrote:

> On 8 December 2014 at 09:16, Bharata B Rao <bharata.rao@gmail.com> wrote:
>> So this approach of parking the vcpu fd in QEMU and reusing it next
>> time during CPU hotplug will only work if all architectures supported
>> by KVM driver are ok with reuse of vcpu object in the kernel. I am
>> using relevant bits of this patch of yours with PowerPC architecture
>> to support cpu unplug and I realize that there is at least one case
>> where reuse of vcpu object in the kernel causes problems. Before I
>> attempt to fix that in ppc KVM, I wanted to check if there is
>> consensus on this approach (parking and reusing kvm vcpu fd) in QEMU.
> 
> For ARM we've just clarified the kernel-to-userspace ABI, and
> among other things it says you can't reuse the VCPU unless it's
> for exactly the same CPU type and feature configuration. So

Yes, it's a limitation now, but it is not the final implementation.
As to the QEMU side, the vcpu object in kernel is stateless, and a
reset operation will be done when we hot add a vcpu, so I think we
can kill the limitation, just choose a parked vcpu and reconfigure
it before we use it.

Thanks,
Gu 

> I'm not sure this park-and-reuse approach will work...

> 

> -- PMM
> .
>