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

>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Jeremy Fitzhardinge
>Sent: Wednesday, May 12, 2010 1:40 AM
>To: Ian Campbell
>Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Jan Beulich
>Subject: Re: [Xen-devel] cpu_down() but no cpu_up() in 
>drivers/xen/cpu_hotplug.c ?
>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@xxxxxxxxxx>
>>     Acked-by: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
>>     Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
>> 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?).

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.


>> 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
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