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

[jim burns <jim_burn@xxxxxxxxxxxxx>]

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 
xen-aware.

> 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. ÂIt includes
>  > > paravirtualisation for memory, SMP, etc as well as paravirtualised
>  > > network and block devices. ÂIt 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! ÂParavirt
> guests can't use the emulated framebuffer that HVM uses, they have to use a
> paravirt framebuffer if they want to output graphically. ÂThe 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.

Don't know what other distros are doing w/ pvops kernels as far as frame 
buffers go.

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