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Re: [Xen-devel] Hypervisor architecture?

To: Etienne Martineau <etmartin@xxxxxxxxx>
Subject: Re: [Xen-devel] Hypervisor architecture?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 04 May 2010 13:33:02 -0700
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On 05/04/2010 12:25 PM, Etienne Martineau wrote:
> Is there any documents around that describe the 'internal' architecture
> of the Hypervisor?
> So far my understanding is such that the Hypervisor is a 'strip-down'
> Linux 2.6.(12/13) + lot's of customization specific to Xen. 

Xen and Linux diverged a long time ago, early in the Linux 2.6 series,
with occasional feature-specific transfusions from Linux into Xen
since.  There are some similarities in how they deal with platform
issues (APICs, etc), but they're mostly different now.

> What is common?
> What is different?

A vcpu is akin to a task; a domain is like a process (where multi-vcpu =
multi-threaded).  Events are a bit like signals (pending events are
checked for on return to guest mode and the return is redirected to the
event handler).  Hypercalls are very similar to syscalls.  Memory
management is largely different.

One of the big differences is that Xen doesn't have a per-vcpu
hypervisor (kernel) stack, and vcpus don't have a hypervisor context. 
While they're actually running in the hypervisor they use a pcpu stack,
but if it blocks/deschedules then it must always return to guest
context, saving away enough info to continue what it was doing when it
re-enters Xen.  Once you get your head around that, a lot of things
become much clearer.


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