RE: [Xen-devel] [Question] vcpu-set before or after xen_pause_requested

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

Ian, thanks for your reply. Yes, I think the intermediate values is not 
important in general. If it is really important, the caller of "xm  vcpu-set" 
should take care of it, i.e. do xm vcpu-set, wait and check the vcpu's status 
(maybe from xen hypervisor) with timeout mechanism.

BTW, from the followed changeset in dom0's drivers/xen/cpu_hotplug.c , it means 
the xm vcpu-set may happen even before guest is fully up, at that time, it is 
difficult for xend to achieve the close loop.

commit d745562cc40bff60f0597004d8128fa0225cc267
Author: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date:   Thu Apr 2 13:24:28 2009 +0100

    xen: honour VCPU availability on boot

    If a VM is booted with offline VCPUs then unplug them during boot. 
Determining
    the availability of a VCPU requires access to XenStore which is not 
available
    at the point smp_prepare_cpus() is called, therefore we bring up all VCPUS
    initially and unplug the offline ones as soon as XenStore becomes available.

    Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

Thanks
--jyh

>-----Original Message-----
>From: Ian Jackson [mailto:Ian.Jackson@xxxxxxxxxxxxx]
>Sent: Wednesday, April 28, 2010 6:31 PM
>To: Jiang, Yunhong
>Cc: Liu, Jinsong; xen-devel; Keir Fraser; Zhai, Edwin
>Subject: RE: [Xen-devel] [Question] vcpu-set before or after 
>xen_pause_requested
>
>Jiang, Yunhong writes ("RE: [Xen-devel] [Question] vcpu-set before or after
>xen_pause_requested"):
>> For example, considering user do vcpu-set 2 (i.e set vcpu number to
>> 2), vcpu-set 4, and vcpu-set 2, and guest didn't take any action at
>> all, would it be really so important? Or, if it is 2->4->3, should
>> the xm vcpu-set itself really need care 4 vcpu is bringup in some
>> time?
>
>This is a good point.  If you are sure that intermediate values are
>not important, and that changes are idempotent, then the open loop
>approach is quite fine.
>
>Ian.

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