RE: [Xen-users] Operative systems supported in virtual machines by Xen

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: nicola [mailto:nicola.calligaris@xxxxxxxx] 
> Sent: 30 May 2007 15:35
> To: Petersson, Mats
> Subject: R: [Xen-users] Operative systems supported in 
> virtual machines by Xen
> 
> Thanks for your help but I don't know 
> Xen (I've used only VMWare for virtualization) and my question is:
> 
> I've one server where I install Fedora Core 6
> with kernel modified for Xen supporting.
> Can I install then virtual machines on Fedora Core 6 server
> with, for example, RedHat 3 ES, Windows 2003 Server and Fedora Core 6?
> (So I have 3 server on the same hardware for my production
> enviroenment).
> 
> I don't know the HVM concept, sorry, can you explain me?

HVM, aka "Full Virtualization" makes use of the virtualization extensions that 
are available in current models of AMD/Intel processors (AMD-V, SVM or Pacifica 
and Intel VT, Vanderpool respectively), which allows the processor to have two 
"kernel modes" at the same time, one for the hypervisor (Xen) and one for the 
guest (e.g. Windows). The hypervisor also asks the processor to stop the guest 
when it does certain operations, e.g. updating the CR3 register, so that the 
hypervisor can "protect itself" and "lie" to the guest about things that the 
guest need not know about (such as the fact that the guest isn't alone, 
physical memory doesn't start at zero, and hardware devices doesn't EXIST as 
hardware). 

This is in contrast to "Para-virtualization", which is when you take a OS 
kernel and modify it to match with the hypervisor. This is of course only 
"easy" if you have the source-code of the OS, so Linux is easy, Windows isn't. 
The advantages are several: It's faster (because the hypervisor don't have to 
"intercept" everything all the time, we can "trust" the guest to know things 
like memory-not-starting-at-zero situations and virtual devices) and it runs on 
any hardware that runs the original OS. But it's also more intrusive (there are 
a huge number of changes to the source-code, and thus the binary code of the 
OS, where code is added/changed compared to the original code, bugs may be 
introduced). 

If you have a HVM-cabale processor (e.g. a Socket F/AM2 Opteron/Athlon64 or 
recent Intel processor models), then you can install Windows as a virtual 
machine, and any Linux-based guest will also work fine in this mode. If not, 
you're stuck with para-virtual domains, which use a "modified" Linux kernel, 
and this also means a more limited selection of Kernels. 

Also, re-reading your question [and I may be trying to find a more complicated 
question than you intended], you seem to ask if you can install Xen inside Xen, 
which in general isn't a good idea, although it is possible to run a Xenified 
Linux as a HVM-guest on Xen. The guest can't run HVM-guests tho'. 

--
Mats
> 
> Best regards
> 
> Nicola
> 
> 
> 
> -----Messaggio originale-----
> Da: Petersson, Mats [mailto:Mats.Petersson@xxxxxxx] 
> Inviato: mercoledì 30 maggio 2007 16.18
> A: nicola; xen-users@xxxxxxxxxxxxxxxxxxx
> Oggetto: RE: [Xen-users] Operative systems supported in 
> virtual machines
> by Xen
> 
>  
> 
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> > [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of nicola
> > Sent: 30 May 2007 15:11
> > To: xen-users@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-users] Operative systems supported in virtual 
> > machines by Xen
> > 
> > Hi,
> > 
> > my question is: which operative systems in the virtual 
> > machines are supported by Xen 3.0 ?
> > For example: can I have a Fedora Core 6 with Xen 3.0 and 
> > create 3 virtual machine whith Windows Server 2003, RedHat 
> > ES5 and Ubuntu respectively installed for production enviroenment?
> 
> Assuming your processor has HVM (either AMD-V or Intel VT), 
> then you can
> run "almost any operating system". On AMD systems, we test regularly
> with Redhat, SuSE and various versions of Windows. I'm fairly 
> sure that
> Intel does the same, but I can't really say for sure.
> 
> I have personally run Windows 2K3, Vista, NT, SuSE SLES 9.3, 
> DOS 6.22 in
> HVM mode. Others have managed to get for example OS/2 to run on AMD
> processors too. 
> 
> If you are using a non-HVM model of processor, you won't be 
> able to run
> Windows. Linux varieties should still work just fine (except you don't
> get the same kernel as the original distribution - you'll be using the
> Xenified kernel version, so if you plan to use some old RedHat version
> that was built with a 2.4 or 2.2 kernel, it may not work well with a
> 2.6.18 kernel that the latest Xen uses... 
> 
> 
> --
> Mats
> 
> 
> 
> 
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> 



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