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

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: [Xen-devel] HVM vcpu hotplug (was: shutdown/reboot/sysrq/vcpu-set from dom0 to HVM guest)
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Thu, 2 Sep 2010 14:21:52 +0100
Cc: Frédéric VANNIÈRE <frederic@xxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jeremy, Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>
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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

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