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Re: [Xen-devel] HVM vcpu hotplug (was: shutdown/reboot/sysrq/vcpu-set fr

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] HVM vcpu hotplug (was: shutdown/reboot/sysrq/vcpu-set from dom0 to HVM guest)
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Thu, 2 Sep 2010 14:27:34 +0100
Cc: Frédéric VANNIÈRE <frederic@xxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>
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On Thu, 2010-09-02 at 14:21 +0100, Stefano Stabellini wrote:
> On Wed, 1 Sep 2010, Stefano Stabellini wrote:
> > > 
> > > For the cpu-set I've modified the file 
> > > linux-2.6.36-rc3/drivers/xen/cpu_hotplug.c and the check fo pv_domain in 
> > > setup_vcpu_hotlug_event(void).  And then I can remove CPUs but not add 
> > > some  :
> > > 
> > >     xl vcpu-set DOMID 2 -> "CPU 2 is now offline / CPU 3 is now offline" 
> > > 
> > > I've tried the same modification on ballooning driver and hvc console one 
> > > but it gave me a kernel panic ...
> > > 
> > 
> > In my tree both the ballooning driver and the hvc console should work
> > correctly on hvm.
> > Regarding the pv cpu hotplug mechanism, I don't think is a good idea to
> > enable it on hvm: there must be an alternative fully virtualized way to
> > hotplug a cpu into a non-pv guest, like Windows, and that should work
> > for pv on hvm guests too.
> I have done some testing on cpu hotplug for hvm guests and it turns out
> that it is currently broken on xen-unstable.
> I have just sent two patches to fix it.
> Once they got applied you should be able to specify the maximum number
> of vcpus using the "maxvcpus" parameter in your config file; then use
> vcpu-set to dynamically change the number of vcpus in the guest.
> Linux needs to be compiled with CONFIG_ACPI_HOTPLUG_CPU=y in order for
> this to work.
> Once you hotplug a new cpu in the guest you also need to enable it
> manually, with the following command:
> echo -n 1 > /sys/devices/system/cpu/cpu$CPUNUMBER/online

FWIW putting this in /etc/udev/rules.d/ somewhere can automate this for

        ACTION=="add", SUBSYSTEM=="cpu", RUN+="/bin/sh -c '[ ! -e 
/sys$devpath/online ] || echo 1 > /sys$devpath/online'" 


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