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Re: [Xen-devel] dom0-cpus problem with Xen 3.4.1-rc6

To: Pasi Kärkkäinen <pasik@xxxxxx>, Nerijus Narmontas <n.narmontas@xxxxxxxxx>
Subject: Re: [Xen-devel] dom0-cpus problem with Xen 3.4.1-rc6
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 22 Jul 2009 18:15:05 +0100
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On 22/07/2009 18:01, "Pasi Kärkkäinen" <pasik@xxxxxx> wrote:

>> Can you tell me what's the difference between Xen boot
>> option dom0_max_vcpus=1 and (dom0-cpus 1) option
>> in /etc/xen/xend-config.sxp?
> If I haven't misunderstood this dom0-cpus option in xend-config.sxp tells
> which physical CPUs/cores vcpu's of dom0 will use..
> ie. if you limit dom0_max_vcpus=1, then you can use dom0-cpus to tell which
> one of the 8 available cpus/cores dom0's 1 vcpu will run on.

dom0-cpus does the same as dom0_max_vcpus -- it specifies number of VCPUs
which dom0 should run with. The difference is that dom0_max_vcpus=1 means
that is all that dom0 kernel will detect and boot with: you cannot
subsequently enable more. With (dom0-cpus 1), dom0 will boot with a vcpu for
every host cpu (by default) and then hot-unplug/offline all but one vcpu
when xend starts. The latter is obviously a more complex operation, but
could be reverted (i.e., you could online some of those vcpus at a later

 -- Keir

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