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In response to Dario's question,

x86 Virtualization now exists on most modern hardware, and provides the infrastructure needed to map certain hardware components such as RAM and CPU's to a virtual machine with near-native performance. However since some devices do not yet have such infrastructure in place they must operate through emulated layers.

Since before this technology, para-virtualization has allowed us to avoid emulating these layers by telling the Guest it is sitting on a hypervisor and giving it more direct access to components, which bypasses the emulated layers and improves performance. This generally requires specific drivers and support from that guest OS. Moving a guest with paravirtualization from one set of hardware to another can create undesirable results ranging from mildly annoying driver installation to system breaking compatibility problems.

Full Virtualization uses a combination of virtualized (mapped) components and emulated layers where such infrastructure does not exist, which means the guest is entirely unaware of the actual physical hardware on the system, and you can transplant that guest on any set of hardware running the same hypervisor without risk of the guest being unable to function due to missing drivers.

Hence, para-virtualization provides greater performance (by bypassing layered operations) at the cost of compatibility (since it now uses drivers specific to the physical hardware on the server).

Hope that helps, and please make corrections if I got anything wrong.

~Casey

On Mon, Jun 11, 2012 at 10:18 AM, Dario Faggioli <raistlin@xxxxxxxx> wrote:

On Sat, 2012-06-09 at 11:03 +0800, Niu Xinli wrote:
> Hi,

>
Hi,

> Para-virtualization outperforms full-virtualization at the cost of
> compatibility.
>

So, if I can ask, do you mind explaining a bit more what you mean with
this sentence above? More specifically, when you say "at the cost of
compatibility", what are the compatibility costs you are referring to?

Just out of curiosity and for the benefit of the on-going
discussion. :-)

Thanks and Regards,
Dario

--
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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Re: [Xen-users] The future of para-virtualization