[CentOS-virt] Xen PV domU reported as Xen-HVM

[Karl Johnson]

On Tue, Jun 17, 2014 at 5:36 AM, George Dunlap <dunlapg at umich.edu> wrote:

> On Thu, Jun 12, 2014 at 11:23 PM, Karl Johnson <karljohnson.it at gmail.com>
> wrote:
> > Hello,
> >
> > I am running two dom0s, one on CentOS 5 with Xen 4.1.2 (from Gitco) and
> the
> > other one on CentOS 6 with Xen 4.2.4 (from Xen4CentOS). I host one LVM
> based
> > domU on both from the same template (CentOS 6 PV) with the same Xen
> config
> > (see below). However, the domU on Xen 4.1 reports itself as Xen PV while
> the
> > domU on Xen4CentOS reports itself as Xen HVM.
> >
> >
> > == First domU ==
> >
> > virt-what and cPanel scripts report domU as: xenpv
> >
> > dom0: 2.6.18-371.6.1.el5xen Xen 4.1.2
> >
> > domU Config:
> >
> > bootloader = '/usr/bin/pygrub'
> > extra      = ''
> > memory     = 6144
> > hostname   = 'web2.x.ca'
> > cpu_weight = 256
> > name       = 'vm115'
> > vif        = ['ip=205.X.X.X, vifname=vifvm115.0, mac=00:16:3e:7a:X:X']
> > vnc        = 0
> > vcpus      = 6
> > vncviewer  = 0
> > serial     = 'pty'
> > disk       = ['phy:/dev/xen/vm115_img,xvda1,w',
> > 'phy:/dev/xen/vm115_swap,xvda2,w']
> >
> > domU kernel: 2.6.32-431.5.1.el6.x86_64
> >
> > root at web2 [~]# virt-what
> > xen
> > root at web2 [~]#
> >
> >
> >
> > == Second domU ==
> >
> > virt-what and cPanel scripts report domU as: xenhvm
> >
> > dom0: 3.10.32-11.el6.centos.alt.x86_64  Xen 4.2.4-29.el6
> >
> > domU Config:
> >
> > bootloader = '/usr/bin/pygrub'
> > extra      = ''
> > memory     = 6144
> > hostname   = 'web3.x.ca'
> > cpu_weight = 256
> > name       = 'vm116'
> > vif        = ['ip=205.X.X.X, vifname=vifvm116.0, mac=00:16:3e:cf:X:X']
> > vnc        = 0
> > vcpus      = 6
> > vncviewer  = 0
> > serial     = 'pty'
> > disk       = ['phy:/dev/xen/vm116_img,xvda1,w',
> > 'phy:/dev/xen/vm116_swap,xvda2,w']
> >
> > domU kernel: 2.6.32-431.5.1.el6.x86_64
> >
> > root at web3 [~]# virt-what
> > xen
> > xen-hvm
> > root at web3 [~]#
> >
> > Any idea why?
>
> A lot of these tools use quirks of the hypervisor to detect these
> kinds of changes.  One such quirk in Xen PV is handling of the CPUID
> instruction: normally there's no way to trap the CPUID instruction in
> PV mode, so Xen introduced a special PV CPUID (which is just an
> illegal instruction that Xen recognizes and emulates itself)  that the
> kernel is supposed to use instead; but that's an opt-in thing -- you
> can still execute the normal CPUID.  By executing the PV CPUID and the
> normal CPUID and comparing the results, you can tell if you're in a PV
> or an HVM VM.
>
> However, on recent architectures Intel has introduced functionality to
> allow trapping os CPUID even in non-HVM mode.  Support for this went
> into 4.2.  Now, if you have the right hardware, and the right Xen
> version, you'll get the same results whether you execute the PV CPUID
> or the native one.
>
> So virt-what's trick of comparing the two CPUID results to determine
> if you're in a PV or an HVM domain doesn't work anymore.
>
>  -George
>

Thanks for the reply George. Is there any other easy way in the domU to
know if it's a pv or hvm with Xen 4.2+ and the right hardware?

Karl
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