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Re: [Xen-users] paravirt_ops DomU, does it mean performance?

  • To: "jim burns" <jim_burn@xxxxxxxxxxxxx>
  • From: "Emre ERENOGLU" <erenoglu@xxxxxxxxx>
  • Date: Mon, 26 May 2008 11:02:23 +0200
  • Cc: xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 26 May 2008 02:02:57 -0700
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Thanks Jim,

On Mon, May 26, 2008 at 4:20 AM, jim burns <jim_burn@xxxxxxxxxxxxx> wrote:
On Sunday May 25 2008 09:28:21 pm Emre ERENOGLU wrote:
> I'm again a bit confused. I'm trying to boot up a 2.6.24 kernel on a HVM
> DomU, and the kernel tells me "booting paravirtualized kernel on bare
> hardware", but I can see no paravirtual devices in /dev.
> Which devices shall it be? What are the names of the modules? Is there a
> clear name for them or does each distro choose a name for itself?
> I found names starting with virtio_* but I'm not sure if they are the
> correct ones, at least in this 2.6.24 (from development version of Pardus),
> modprobbing them gives no output in dmesg nor any devices in /dev
> Or maybe we need to put some special configutation options in the config
> file of the HVM domU?

Hvm won't get performance w/o something like the unmodified drivers you
installed awhile back. Hvm domus *think* they are 'booting [...] on bare
hardware', and are not aware that they are running under a hypervisor. The pv
drivers do that. Pvops is just alternative approach to standard xen code to
running under a hypervisor as a pv domu. It doesn't make an hvm domain

Got it.  I can load the PV drivers up, but I was too optimistic to think that the pv_ops would automatically bring some PV drivers in place and especially after seeing that kernel message in dmesg. But dissapointed now with your explanation.

> On Wed, Dec 26, 2007 at 4:43 PM, Mark Williamson
<mark.williamson@xxxxxxxxxxxx> wrote:
> > > The paravirt_ops kernel should run with similar performance to the
> > >
>  > > XenLinux kernel from the main Xen tree.  It includes
>  > > paravirtualisation for memory, SMP, etc as well as paravirtualised
>  > > network and block devices.  It doesn't include support for
>  > > suspend/resume, paravirt framebuffer, etc.
>  >
>  > When you say 'paravirt framebuffer', do you mean console performance is
>  > going to be as bad as for HVM?
> Well, more that it doesn't have a graphical console at all!  Paravirt
> guests can't use the emulated framebuffer that HVM uses, they have to use a
> paravirt framebuffer if they want to output graphically.  The paravirt
> framebuffer doesn't perform significantly better than the HVM console as
> far as I know, so not having it available may not be too painful to you...

I've mentioned before that what Fedora 9 (2.6.25 kernel) does is use the
standard Xorg fbdev module in a pv domain, but it uses 'qemu-dm -M xenpv ...'
to run the domain. (Actually, Fedora has been doing that since F8, fully
patched, not dvd version.) Even their non-xen 2.6.25 kernel won't boot in a
pv domu. It doesn't enable the xen frontends in the kernel config, but even
when I've tried to do that, the kernel won't boot. (See the thread 'Creating
an fc8 2.6.23 domu' in late Feb., early Mar.) Apparently there is still going
to be a lot of work to merge xen into mainline.

I don't understand. Mark says that paravirt_ops kernel shall run at near performance to XenLinux kernel, but you say that it's not possible to boot such kernel. My experience is the same, I couldn't boot this 2.6.24 as a PV domU. So where do we use this pv_ops?

My understanding was, given the necessary KERNEL_OPTIONS are enabled, the kernel shall boot fine as a Xen PV DomU, thus it would have been enough to write this inside the domu config file file kernel= line???

Emre Erenoglu

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