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Message-ID: <4A59E3E1.30406@codemonkey.ws>
Date: Sun, 12 Jul 2009 08:23:45 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [patch 2/2] QEMU BOCHS bios patches to use maxcpus
	value.
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To: Avi Kivity <avi@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Jes Sorensen <jes@sgi.com>, qemu-devel@nongnu.org

Avi Kivity wrote:
> On 07/10/2009 12:57 AM, Anthony Liguori wrote:
>> These changes make my Ubuntu server guest very unhappy.  I get a 
>> bunch of messages about "Not responding." on startup.
>>
>> If nothing else, maxcpus ==smp_cpus under QEMU because we don't do 
>> CPU hotplug (and I don't think we should).
>>
>
> Why shouldn't we do cpu hotplug?

I don't think we should do cpu hotplug via ACPI.  I don't think ACPI 
actually models CPU hotplug and the fact that this works with Linux in 
KVM is a happy accident.  VMware only supports CPU hotplug for Windows 
7/2k8 guests so I'm assuming their using Viridian PV extensions to do it.

I think we should go the PV route for Linux too.  I'd rather see us 
create all vcpu threads at once and then let the guest offline each vcpu 
via a PV notification.  I don't see a lot of value in 
spawning/terminating vcpu threads dynamically and it adds an awful lot 
of complexity.  There's very little overhead to an idle thread.

Regards,

Anthony Liguori