Changes between Version 15 and Version 16 of VirtualBox architecture

Timestamp:

Nov 16, 2006 5:31:29 PM (17 years ago)

Author:

jose

Comment:

--

VirtualBox architecture

@@ -58 +58 @@
    * ring 3 (hopefully executed in "raw mode" all the time);
    * ring 1 (of which the guest thinks it's ring 0, see above), and 
-   * ring 0 (which is !VirtualBox code). This guest-context ring-0 code is also often called a "hypervisor". 
+   * ring 0 (which is !VirtualBox code). This guest-context ring-0 code is also often called a '''"hypervisor".'''
 
 Finally, there is also a ring-0 driver that must be loaded in the host OS for !VirtualBox to work. However, this ring-0 driver does less than you may think. It is only needed for a few specific tasks, such as:
@@ -75 +75 @@
 == Intel VMX ("Vanderpool") support == 
 
-With its latest processors, Intel has introduced hardware virtualization support 
+With its latest processors, Intel has introduced hardware virtualization support, which they call "Vanderpool", "IVT", "VT-x", or "VMX" (for "virtual machine extensions"). We use the term "VMX". A thorough explanation of this architecture can be found on [http://www.intel.com/technology/itj/2006/v10i3/1-hardware/5-architecture.htm Intel's pages], but as a summary, with these extensions, a processor always operates in one of the following two modes:
+
+ * In '''root mode''', its behavior is very similar to the standard mode of operation (without VMX), and this is the context that a virtual machine monitor (VMM) runs in.
+ * The '''non-root mode''' (or guest context, if you want) is designed for running a virtual machine.
+
+The notable novelty is that all four privilege levels (rings) are supported in either mode, so guest software can theoretically run at any of them. 
+
+Since, however, nearly all operating systems in use today only make use of ring-0 and ring-3, and since context switching between root mode and non-root mode is very expensive, !VirtualBox does not use VMX entirely as intended by Intel. Instead, we make partial use of it -- only where it makes sense and where it helps us to improve performance.
+
+