>-----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.
--jyh
>
>> 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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