From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matt Wilson <msw@amazon.com>
Subject: Re: Xen 4.2 Release Plan / TODO
Date: Tue, 26 Jun 2012 15:57:00 -0700
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To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>, xen-devel <xen-devel@lists.xen.org>
List-Id: xen-devel@lists.xenproject.org

On Tue, Jun 26, 2012 at 02:09:06PM -0700, Konrad Rzeszutek Wilk wrote:
> On Tue, Jun 26, 2012 at 01:31:51PM -0700, Matt Wilson wrote:
> > On Tue, Jun 26, 2012 at 01:39:42AM -0700, Ian Campbell wrote:
> > >     * [BUG] vcpu-set doesn't take effect on guest, reported by Intel:
> > >       http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1822
> > 
> > This may be a problem with the guest, rather than a problem with Xen
> > or the toolstack. I tested on a domU running a 3.2.20-based Linux
> > kernel. The domU kernel didn't automatically enable the vCPUs after
> > calling "xl vcpu-set" to increase the allocation.
> > [...]
> > 
> > I've added this as a comment to the bugzilla.
> 
> That is a generic bug - its a race in the generic code where
> the cpuX directory is created; then udev kicks off and tries to
> echo 1 > cpuX/online (but online hasn't been created yet); the cpu-hotplug
> code creates 'online' directory.
> 
> I've asked Greg KH about it, and here is his take:
> https://lkml.org/lkml/2012/4/30/198
> 
> But I never got to prep a patch. If somebody gets to do it before me,
> I owe them a beer!

My problem was that I didn't even have a udev rule to auto-online
hotplugged CPUs. Everything works as expected after adding the rule
suggested on the wiki: http://wiki.xen.org/wiki/Paravirt_Linux_CPU_Hotplug

I'm thinking that the bug report is just this well hashed out
problem. For historical reference, here's the thread discussing the
behavior change in pv-ops kernels:
  http://lists.xen.org/archives/html/xen-devel/2010-05/msg00516.html

Matt