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From: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Date: Mon, 26 Jan 2015 10:01:48 +0800
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Subject: Re: [Qemu-devel] [PATCH v2 00/11] cpu: add i386 cpu hot remove
 support
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To: Alexandre DERUMIER <aderumier@odiso.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, tangchen@cn.fujitsu.com, chen fan fnst <chen.fan.fnst@cn.fujitsu.com>, isimatu yasuaki <isimatu.yasuaki@jp.fujitsu.com>, Igor Mammedov <imammedo@redhat.com>, guz fnst <guz.fnst@cn.fujitsu.com>, Anshul Makkar <anshul.makkar@profitbricks.com>, afaerber <afaerber@suse.de>

On Fri, 2015-01-23 at 11:24 +0100, Alexandre DERUMIER wrote:
> Hello,
> 
> I'm currently testing the new cpu unplug features,
> Works fine here with debian guests and kernel 3.14.
> 

Thanks for your test.

> But I have notice some small potential bugs, but I'm not sure I'm doing it right.
> 
> 1)first, to unplug cpu, we need an id for cpu
> 

Yes, if you want to unplug cpu, you must have an id for cpu.

> The problem is that the current qemu command line
> -smp 1,sockets=2,cores=1,maxcpus=2
> 
> for example, will create 1 cpu on apic-id 0  without any id, so we can't unplug it.
> 
> 
> So, I have tried with
> 
> -smp 1,sockets=2,cores=1,maxcpus=2 -device kvm64-x86_64-cpu,apic-id=0,id=cpu0
> 
> But this give me an error:
> "-device kvm64-x86_64-cpu,apic-id=0,id=cpu0: CPU with APIC ID 0 exists"
> 

APIC ID 0 was used by the cpu of '-smp 1'.
So you should use apic-id=1

> (also try to set -smp 0, but it's not working).
> 
> 
> 
> 2) second problem, if I start with
> -smp 1,sockets=2,cores=1,maxcpus=2
> 
> then hmp: 
> device_add kvm64-x86_64-cpu,apic-id=1,id=cpu1
> 
> then hmp : device_del cpu1
> 
> Got an error:"
> This is the last cpu, should not be removed!"
> 
> 

Oh, it's our problem, thanks for your pointing out.
I will fix it in next version.

Regards,
Zhu

> 
> This is coming from
> [PATCH 06/12] pc: add cpu hot unplug request callback	support
> +    if (smp_cpus == 1) {
> +        error_setg(&local_err,
> +                   "This is the last cpu, should not be removed!");
> +        goto out;
> +    }
> 
> 
> 
> So, the only way unplug is working for me, is to start with -smp 2 minimum
> -smp 2,sockets=2,cores=1,maxcpus=4
> 
> Then I can hotplug|unplug cpuid >= 2
> 
> 
> 
> Regards,
> 
> Alexandre Derumier
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