> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Jordi Segues
> Sent: 30 January 2007 16:56
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Config difference: Paravirtualization
> and full virtualization
>
> Hello,
>
> I'm playing with xen, but I have a doubt:
>
> Where's the difference in the guest installation (or maybe in the host
> installation?) when you install a Linux in paravirtualization mode, or
> in full virtualization mode (VT or pacifica)?
The kernel in Paravirtual (PV) mode is modified. In full-virtualization
(hvm), you just run the standard OS kernel as it comes from whoever
supplies the OS binaries.
>
> I mean, how do you tell to xen that you want to run a linux guest in
> paravirtualization or in full virtualization mode?
When you specify the "kernel=..." line in the configuration, a full
virtualized kernel takes "hvmloader" as a kernel (which it is, in a
strange sort of way - it's a BIOS image + some code to start off at the
"reset vector").
You also specify a "builder=hvm"
When running a paravirtual OS, you specify the Linux (usually) kernel
that you want to boot.
Here you'd specify "builder=linux".
[There may be other "builder" options too]
If you compare /etc/xen/xmexample1 with /etc/xen/xmexample.hvm, you'll
see the difference.
>
> And then, can you run a linux (modified to run under
> paravirtualization) in full virtualization mode?
No, you can't do that - the PV kernel isn't a "regular" kernel, so it
"knows" that it should call Xen to do certain things, which of course it
can't do when it's fully virtualized - the full virtualized kernel will
never know[1] that there is a hypervisor there, and has no ability to
call Xen in the same way that PV kernel does.
[1] Yes, it's technically possible to figure out, not only if it IS a
hypervisor below the OS, but also which hypervisor (Xen or VMWare for
example) by using a special CPUID leaf. This leaf is not used by any
real CPUs, but it's intercepted by the CPUID code in the hypervisor and
given results by the hypervisor - but that assumes the kernel is aware
of this and tries to do so. Which is not normally the case!
>
> Can I run a linux in paravirtualization mode and another in full
> virtualization mode, both simultanously?
Yes - assuming you have at least two procesor cores in the system, you
can run them REALLY simultaneously, but of course, you may in that case
have three domains (Dom0 and two guests), so one may not run at the
exact same moment as the other [I'm being very pedantic with the word
simultaneously here]. But you can certainly have two domains of
different types (HVM/PV) started in the same machine at the same time.
>
> Thanks a lot!
> And sorry if this is a basic question.
>
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