Re: apt-get install xen-tools recursion

["Manfred Haertel, DB3HM" <Manfred.Haertel@xxxxxxxxxxxx>]

Elliott Mitchell schrieb:

> > Recursive virtualization is actually supported under Xen and it works with
> > some possible glitches. I am running Xen under Xen and I once did run Xen
> > under VMware workstation and after solving the glitches without problems.
> Proper hypervisor on hypervisor should be indistinguishable from
> hypervisor on "bare metal".  Similarly an OS running on a hypervisor
> *shouldn't* be able to tell the hypervisor is there.  x86 makes this
> extremely difficult since many instructions behave differently in the
> different modes.  Certainly there are workarounds, but those glitches
> shouldn't be present in the first place.
What do you mean by "many instructions behave differently in the 
different modes"?
"Hardware virtualization" is actually "hardware ASSISTED 
virtualization". Inside a VM, the processor generates traps into the 
hypervisor on certain instructions, so that the hypervisor can do its work.
When using recursive virtualization, the virtualization instructions 
like VMLAUNCH, VMRESUME on the x86 generate a trap which must be handled 
by the hypervisor. This is how recursive virtualization works, handling 
these instructions. So recursive virtualization can never be 
indistinguishable from normal virtualization.
The glitches exist mainly because recursive virtualization isn't as 
widely used as normal virtualization and thus it is also not as 
thoroughly tested as normal virtualization. Using Xen, it works quite 
good under the given circumstances.
--
Manfred Härtel, DB3HM    mailto:Manfred.Haertel@xxxxxxxxxxxx
                         http://rz-home.de/mhaertel