[PLUG] Virtual Iron

Andy Grover andy at groveronline.com
Mon Oct 22 05:23:45 UTC 2007


Michael Ewan wrote:
> What Intel VT gives you is the ability to run an unpatched OS in a 
> para-virtualized environment i.e. Xen.

Xen has 2 modes for guests: paravirtualized and fully-virtualized (aka
HVM). So it is correct to say you can run an unpatched OS under Xen with
VT, but it is not running paravirtualized, it's in HVM mode.

> Without hardware support for
> virtualization, you cannot run an out-of-the-box OS (Windows,
> unpatched Linux, etc) as a virtual client OS.

Well you *can* but it's really hard, which is why VMWare were really the
only ones who had it working well before VT and SVM were introduced. Now
everyone's doing it because proper CPU support makes it so much easier :)

> VT will give you a performance boost over full hardware abstraction,
> since your are using the system resources directly rather than
> multiple layers of virtual drivers.

Xen HVM guests are not particularly fast, because VT lets Xen fully
virtualize the CPU, but device accesses require emulation and are slow,
most notably network and block accesses. This is the problem that PV
guest drivers like "VMWare Tools" fix -- the OS is unmodified, but
things are sped up by adding some drivers that know how to send data to
the hypervisor directly instead of simulating hardware for the OS's
existing drivers to talk to. Xen also has these -- but only for Linux.
Virtual Iron and others sell PV guest drivers for Windows as part of
their added value.

So:
* VMWare: doesn't need VT, virtualizes any OS the hard way, has PV
drivers, $$$
* Xen: doesn't need VT, can only run on modified OSes, has PV drivers
* Xen HVM: requires VT, virtualizes any OS the easy way, has PV drivers
only for Linux (VI, Novell, XenSource sell Win PV drivers for $$$)

HTH -- Regards -- Andy




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