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Re: [Xen-users] how does hypervisor isolates DomU from Dom0

To: "Tapas Mishra" <mightydreams@xxxxxxxxx>, "Xen List" <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] how does hypervisor isolates DomU from Dom0
From: "Nick Couchman" <Nick.Couchman@xxxxxxxxx>
Date: Mon, 16 Aug 2010 10:51:09 -0600
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>>> On 2010/08/15 at 14:19, Tapas Mishra <mightydreams@xxxxxxxxx> wrote: 
> I am not clear with the working of hypervisor xen.
> How does an instruction executed by kernel in Ring0 of DomU gets
> mapped or executed by hypervisor and then is excuted in Dom0.
> Is this thing some where on the wiki page?

I'm not all that familiar with the internals of Xen, but I think the high-level 
answer to your question is, it depends.  It depends on what type of domU you're 
running.  Paravirtualized domUs are aware that they are virtualized and that 
they do not have ring 0 access, so I'm guessing they do not attempt to execute 
instructions in ring 0 without passing it through to the hypervisor, which 
brokers those requests.  HVM domUs, on the other hand, are full machines (more 
along the lines of how VMware does virtualization) that can run an unmodified 
kernel.  So, kernels in HVM domUs attempt to access ring 0 resources, which 
must then be translated by the HVM and passed on to the hypervisor.

That's my understanding of how it works...


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