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Re: [Xen-devel] cpu_down() but no cpu_up() in drivers/xen/cpu_hotplug.c

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>
Subject: Re: [Xen-devel] cpu_down() but no cpu_up() in drivers/xen/cpu_hotplug.c ?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 12 May 2010 09:26:40 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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On 05/11/2010 08:25 PM, Jiang, Yunhong wrote:
>> Yes, it was to make it consistent with native physical CPU hotplug.  It
>> also replaced some other xen-specific mechanism to allow the domain to
>> control when the cpu was actually added (I forget the details; something
>> like "cpus allowed" vs "cpus active" or something?).
> I remember for cpu remove, the xen's vcpu is different to native method. In 
> native, it will only trigger a uevent to user space (at least in version like 
> 2.6.31), while for xen vcpu, it will remove the vcpu directly.

I would think that Xen and native are much the same; if you pull out a
physical CPU from the system, that's in no way advisory ;)  Similarly,
if you remove a vcpu from a guest, that's an external policy being
imposed onto the guest, and it doesn't get much say in the matter,
beyond being able to clean up before the vcpu goes away.

If the domain wants to stop using a vcpu, it can simply do that by
soft-downing the vcpu, but it remains attached to the domain (again,
analogous to the native case).


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