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Re: [Xen-devel] CPU enumeration in Xen

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] CPU enumeration in Xen
From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
Date: Wed, 22 Aug 2007 14:48:55 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Dominik Klein <dk@xxxxxxxxxxxxxxxx>
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On Wed, Aug 22, 2007 at 01:34:47PM +0100, Keir Fraser wrote:
> It seems a bit unlikely, since the enumeration code is from Linux. Perhaps
> the enumeration order has changed in Linux?
> To my knowledge the enumeration order is determined by the order we find
> CPUs in the BIOS tables, and can hence vary across different systems, and
> even across BIOS upgrades.

The HV may enumerate CPUs in the same way as Linux baremetal, but Dom0 sees
VCPUs not physical CPUs. So /proc/cpuinfo in Dom0 will bear little resemblance
to the same file on a bare metal machine. You'd need to compare the HV's view
of physical CPUs to the Linux bare metal view, which isn't something you can
do  by merely comparing /proc/cpuinfo AFAICT.

> On 22/8/07 11:34, "Dominik Klein" <dk@xxxxxxxxxxxxxxxx> wrote:
> > http://lists.xensource.com/archives/html/xen-users/2007-08/msg00507.html
> > 
> > In this thread I posted some interesting experiment results which lead
> > to the assumption that Xenified Linux does not enumerate CPUs the same
> > way as a non-xen-Linux does.
> > 
> > Can someone confirm this?

Also, the Dom0 sees no (NUMA) topology - all CPUs are considered equal, so 
Dom0 doesn't see any of the hyperthread, cores, sockets, nodes relationship
that a baremetal kernel sees - the HV of course knows about this topology
but doesn't expose it to the guest kernels when assigning them vCPUs.

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