From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: cpu_down() but no cpu_up() in
	drivers/xen/cpu_hotplug.c ?
Date: Tue, 11 May 2010 10:40:15 -0700
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To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, Jan Beulich <JBeulich@novell.com>
List-Id: xen-devel@lists.xenproject.org

On 05/11/2010 02:45 AM, Ian Campbell wrote:
> The original commit which added CPU hotplug to pvops says:
>
>     xen: implement CPU hotplugging
>     
>     Note the changes from 2.6.18-xen CPU hotplugging:
>     
>     A vcpu_down request from the remote admin via Xenbus both hotunplugs the
>     CPU, and disables it by removing it from the cpu_present map, and removing
>     its entry in /sys.
>     
>     A vcpu_up request from the remote admin only re-enables the CPU, and does
>     not immediately bring the CPU up. A udev event is emitted, which can be
>     caught by the user if he wishes to automatically re-up CPUs when available,
>     or implement a more complex policy.
>     
>     Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
>     Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
>     Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
> I'm not sure how the decision was reached to implement it this way,
> perhaps for consistency with CPU hotplug on other
> platforms/architectures?
>   

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?).

> FWIW I use a udev rule to bring up CPUs as they are added, which is
> equivalent to the old behaviour:
>
>         ACTION=="add", SUBSYSTEM=="cpu", RUN+="/bin/sh -c '[ ! -e /sys$devpath/online ] || echo 1 > /sys$devpath/online'"
>   

Fedora and RHEL have been shipping with something like this for a while.

    J