1. Xen and the Art of Virtualization
Paul Barham, Boris Dragovic, Keir Fraser, Steven Hand, Tim Harris, Alex Ho, Rolf Neugebauer, Ian Pratt & Andrew Warfield
Presented by Judy Fischbach

2. CS533 - Concepts of Operating Systems
Overview
Motivations
How Xen works
Comparisons among Xen, hardware, and VMWare.
Xen and VMWare both designed for x86 architecture
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CS533 - Concepts of Operating Systems

3. Virtualization Challenges
Speed
Performance
Security
Resource Isolation
Functionality
Xen’s design goal
The authors came up with the design goal of being able to run 100 simultaneous virtual machine implementations with Binary Compatibility
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CS533 - Concepts of Operating Systems

4. Types of virtualization
Two categories exist
Full Virtualization
Disco and VMWare
Paravirtualization
Xen
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CS533 - Concepts of Operating Systems

5. CS533 - Concepts of Operating Systems
VMM Approach
In traditional VMM (Virtual Machine Monitor) the virtual hardware is functionally equivalent to the physical hardware
Possibly difficult to implement (x86 systems in particular)
Potentially useful to provide real and virtual resource information
The “guest” machine would not have access to this information
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CS533 - Concepts of Operating Systems

6. CS533 - Concepts of Operating Systems
Xen Approach
Virtual machine is NOT 100% functionally equivalent to the hardware
Xen uses paravirtualization instead
Paravirtualization is a process where the guest operating system is modified to run in parallel with other modified systems
Advantage: Improved performance
Disadvantage: The hosted operating system must be modified before it can be hosted by the Xen Hypervisor (can be difficult)
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CS533 - Concepts of Operating Systems

7. CS533 - Concepts of Operating Systems
Xen Design
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8. CS533 - Concepts of Operating Systems
Definitions
Guest Operating System
The operating system that Xen hosts
Domain
The virtual machine under which a guest operating system executes
Guest OS and a domain similar to idea of a program and a process
Hypervisor
A particular Xen version which handles low level functionality
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CS533 - Concepts of Operating Systems

9. CS533 - Concepts of Operating Systems
Xen Implementation
What Xen does differently--
CPU
Virtualization of the CPU
CPU Scheduling
Time & Timers
Memory Management
Virtual Address Translation
Physical Memory
Device I/O
Network
Disk
Control Transfer
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CS533 - Concepts of Operating Systems

10. CS533 - Concepts of Operating Systems
CPU
Most change is required here by the guest OS
Xen takes exception to the basic idea that an OS is the most privileged
Privileged instructions
Instructions are paravirtualized by requiring to be validated/executed within Xen
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CS533 - Concepts of Operating Systems

11. CS533 - Concepts of Operating Systems
CPU Virtualization
Virtualization easier with x86
Why?
Has built in security levels (Rings 0, 1, 2, 3)
Ring 0 – OS Software (most privileged)
Ring 3 – User software
Ring 1 & 2 – Not used
Xen modifies the OS to execute on Ring 1
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CS533 - Concepts of Operating Systems

12. CPU Scheduling and Timers
Borrowed Virtual Time algorithm is used in scheduling domains
Important to lessen problems of one domain executing code that might impact another domain.
Provides several different types of timers
Real Timer
Virtual Timer
Wall Clock Timer
Real Timer - timer that always advances independent from the executing domain
Virtual Timer - timer that advances within the context of the domain
Wall Clock Timer – timer that takes in to account local offsets for time zone and DST
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CS533 - Concepts of Operating Systems

13. CS533 - Concepts of Operating Systems
Control Transfer
Exceptions and Events
Memory faults and software traps
Generally virtualized through Xen’s event handler
Two most frequent exceptions are:
System calls
Page faults
System calls and page faults are two examples of a ‘fast’ handler (one in which doesn’t use the hypervisor)
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CS533 - Concepts of Operating Systems

14. Paravirtualization of the MMU
Full Virtualization
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15. CS533 - Concepts of Operating Systems
Virtual Memory
When the guest OS requires a new page table, it allocates it from its own memory store
After allocation it becomes registered with Xen
OS then gives up all memory write privileges
All subsequent updates must be validated by Xen
Guest OS’s generally batch these update requests to amortize the cost of calling the hypervisor
Segmentation is virtualized likewise
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CS533 - Concepts of Operating Systems

16. CS533 - Concepts of Operating Systems
Virtual Memory
Xen uses a design where
The guest OS’s themselves are responsible for allocation and managing their own hardware pages
Xen itself is located in an unused section at the top of address space to ensure that it is never paged out
This differs from the approach that Disco takes
Disco VMM goes a second level deeper in it’s indirection.
A VMM within VMM
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CS533 - Concepts of Operating Systems

17. CS533 - Concepts of Operating Systems
Memory Management
Memory Management can be challenging
The key points
No software managed TLB exists in x86
The TLB is not tagged which implies it must be flushed on a context switch
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CS533 - Concepts of Operating Systems

18. CS533 - Concepts of Operating Systems
Device I/O Management
Data is transferred to and from domains through a buffer descriptor ring
Based on producer consumer pointers
One set used within guest OS
Another set used within Hypervisor
This allows separation between data and event notification
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CS533 - Concepts of Operating Systems

19. CS533 - Concepts of Operating Systems
Hypervisor Control
In Xen
Domain0 is given greater access to the hardware and the hypervisor. It has:
A guest OS running above the domain
Hypervisor Manager software to manage elements within other existing domains.
In VMWare
Host OS acts underneath the domain.
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CS533 - Concepts of Operating Systems

20. CS533 - Concepts of Operating Systems
Disk I/O Comparisons
Disco
VmWare
Xen
Interface for Disk I/O
Allows the guest OS access to the raw device
Domain0 has direct access to disk and houses virtual block device (VBD) mgmt software
Allows a “virtual disk” creation (binary file in the host OS file system controlled by the VM)
The VBD makes use of the ring mechanism
Subsequent domains confine their disk access through the VBD management software
Maintains tighter control over disk access and allows “batching” of disk requests
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CS533 - Concepts of Operating Systems

21. CS533 - Concepts of Operating Systems
New Domain Creation
Domain0 is privileged
New domains are created by Domain0
Reduces the hypervisor complexity
Allows for a better debug environment
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CS533 - Concepts of Operating Systems

22. CS533 - Concepts of Operating Systems
Networking
Networking in computers are important these days.
Because of this, Xen provides a “Virtual Firewall”
Domain0 creates firewall rules
Data transmission uses a buffer ring for outgoing
Data reception uses a different buffer ring for ingoing
Incoming data packets are analyzed according to Virtual Firewall rules
If any Virtual Firewall rules broken, packet is dropped
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CS533 - Concepts of Operating Systems

23. CS533 - Concepts of Operating Systems
Other Hardware
Block diagram for Xen illustrates that the idea of Xen enabled hardware drivers still exists
VMWare operates similarly.
Two examples:
A sound card is installed, host machine will detect that you have a Sound Blaster enabled sound card.
A video driver is installed on guest OS to improve video performance (from VMWare).
Don’t forget, one more important part of virtualization.
Any other resources that the guest OS can access must be virtualized, not just the memory and cpu!
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CS533 - Concepts of Operating Systems

24. CS533 - Concepts of Operating Systems
Citations
Diagrams from the following paper
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CS533 - Concepts of Operating Systems