1. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Virtualization
Operating Systems, 2017, Danny Hendler & Amnon Meisels

2. What is virtualization?
Creating a virtual version of something
Hardware, operating system, application, network, memory, storage
“The construction of an isomorphism between a guest system and a host” [Popek, Goldberg, ’74]
מה זה בכלל וירטואליזציה? במובן המילולי, וירטואלי פירושו מדומה.
בהקשר של מערכות מחשב, וירטואליזציה מדמה מערכת אורחת –
Guest system
באמצעות מערכת מארחת –
Host system
כאשר כל מצב במערכת האורחת ממופה למצב מקביל במערכת המארחת ולכל מעבר בין שני
מצבים במערכת האורחת יש מעבר מקביל בזו המארחת.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

3. Example: virtual disk
Partition a single hard disk to multiple virtual disks
Virtual disk has virtual tracks & sectors
Implement virtual disk by file
Map between virtual disk and real disk contents
Virtual disk write/read mapped to file write/read in host system
לדוגמא, אנחנו יכולים לחלק דיסק פיסי לכמה דיסקים וירטואליים, כאשר כל דיסק וירטואלי מיוצג על ידי קובץ.
התוכן של הדיסק הוירטואלי נשמר כתוכן של הקובץ וכל פניה לקריאה או כתיבה של בלוק ממופה לקריאה
או כתיבה מהאופסט המתאים בקובץ.
בהקשר של הקורס שלנו, אנחנו נעסוק בוירטואליזציה של חומרה ומערכות הפעלה.

4. What is virtualization? (continued)
A way to run multiple operating systems and applications on the same hardware (virtual machines)
Only virtual machine manager (a.k.a. hypervisor) has full system control
Virtual machines completely isolated from each other (or so we hope)
בהקשר שלנו, הטכנולוגיה של וירטואליזציה מאפשרת ליצור כמה מערכות וירטואליות שאנחנו נקרא להן מכונות וירטואליות, שרצות מעל אותה חומרה כאשר כל אחת מהן יכולה להריץ מערכת הפעלה שונה ו/או להשתמש בשירותים של חומרה שונה. כדי שהמערכות האלו יהיו מבודדות אחת מן השניה, במחשב כזה רצה תוכנת מערכת שנקראת virtual machine manager או hypervisor שרק לה יש גישה מלאה למשאבי המערכת, בעוד המכונות הוירטואליות רצות ברמת הרשאות (privilege) נמוכה יותר. כל מכונה וירטואלית כזו מקבלת נתח ממשאבי המערכת – זמן CPU, main memory, מקום על הדיסק וכולי.
[ועדיין, הפרדה מלאה לגמרי קשה להשיג. למשל, מתברר שמכונה וירטואלית אחת יכולה ללמוד הרבה על האלגוריתמים שרצים על מכונה וירטואלית אחרת באמצעות ניתוח מדויק של דפוסי cache hit/miss שכן המכונות הוירטואליות חולקות באופן דינמי את משאבי ה-cache המשותף.]
Operating Systems, 2017, Danny Hendler & Amnon Meisels

5. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Basic concepts
Virtual Machine (VM)
Host
Guest
Hypervisor (type ||) / Virtual Machine Monitor
כמה מושגים –
Operating Systems, 2017, Danny Hendler & Amnon Meisels

6. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Basic concepts
Virtual Machine (VM)
Host
Guest
Hypervisor (type ||) / Virtual Machine Monitor
כאמור אנחנו יכולים להריץ בו זמנית כמה מכונות וירטואליות
Virtual machine
Operating Systems, 2017, Danny Hendler & Amnon Meisels

7. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Basic concepts
Virtual Machine (VM)
Host
Guest
Hypervisor (type ||) / Virtual Machine Monitor
בהיפרויזור שרץ מעל מערכת ההפעלה (היפרויזור מסוג 2), יש לנו מערכת הפעלה מארחת ויש לנו בכל מכונה וירטואלית מערכת הפעלה אורחת.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

8. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Basic concepts
Virtual Machine (VM)
Host
Guest
Hypervisor (type ||) / Virtual Machine Monitor
Operating Systems, 2017, Danny Hendler & Amnon Meisels

9. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Basic concepts
Virtual Machine (VM)
Host
Guest
Hypervisor (type ||) / Virtual Machine Monitor
Operating Systems, 2017, Danny Hendler & Amnon Meisels

10. Types of virtualization
Full virtualization – guest OS runs unmodified
Para-virtualization – guest OS must be aware of virtualization, source-code modifications required
Hardware virtualization support may be used for both
Our focus is on full virtualization
יש יותר מדרך אחת לבצע וירטואליזציה.
בוירטואליזציה מלאה, מערכת ההפעלה שרצה בתוך המכונה הוירטואלית רצה כמות שהיא, ללא שום שינויים בקוד. לכן, ההיפרויזור צריך ליצור עבורה אשלייה מלאה כך שלמעט אולי ירידה מסויימת בביצועים, היא תרוץ בדיוק כפי שהיתה רצה ישירות על החומרה, למרות שהיא לא רצה ב supervisor mode.
לעומת זאת, כאשר אנחנו יכולים ליצור גירסה שונה של מערכת ההפעלה על ידי התאמה להיפרויזור, אז הגירסה הזו יכולה
לבצע קריאות באופן ישיר לשירותים של ההיפרויזור, מה שיכול לפשט את המימוש, לשפר את הביצועים, ולאפשר
פונקציונליות נוספת (למשל, תקשורת יעילה בין מכונות וירטואליות שונות). מימושים כאלה נקראים para-virtualization.
בארכיטקטורות מודרניות יש גם תמיכה חומרתית בוירטואליזציה (למשל intel/AMD) מה שמאפשר ביצועים משופרים.
אנחנו נתמקד בקורס בוירטואליזציה מלאה.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

11. Virtualization advantages
Cost-effectiveness – less hardware
Multiple virtual machines / operating systems / services on single physical machine (server consolidation)
Various forms of computation as a service
Isolation
Good for security
Great for reliability and recovery: If VM crashes it can be rebooted, does not affect other services (fault containment)
VM migration
Development tool
Work on multiple OS in parallel
Develop and debug OS in user mode
Origins of VMware as a tool for developers
Server consolidation – prior to VMWare ESX server, “IT admins would typically buy, install and configure a new server for every new application/service they had to run in the data center, so servers were very inefficiently utilized and typically used at most 10% of their capacity (during peaks).” [Tanenbaum]
Vmotion – live migration of running VMs over the network (no need to reboot OS or restart apps).
Operating Systems, 2017, Danny Hendler & Amnon Meisels

12. Virtualization vs. Multi-Processing
∙∙∙
User space/ kernel separation OS
HW interface
HW (disk, NIC,…)
Multi-processing VM
Pr1
Pr2
Pr1
Pr2
∙∙∙
מולטיפרוססינג הוא בעצם סוג של וירטואליזציה, וירטואליזציה מוגבלת של פרוססור, כאשר ניתן לחשוב על process כעל מעבד וירטואלי עליו רצה התוכנית. מערכת ההפעלה היא התוכנה שמאפשרת את הוירטואליזציה המוגבלת הזו, היא רצה בkernel mode בעוד התהליכים עצמם רצים בuser space ולא יכולים לגשת ישירות לחומרה אבל כל תהליך מקבל את האשליה שהוא היחיד שרץ על המעבד.
בוירטואליזציה מלאה, מערכת ההפעלה עצמה רצה מעל מה שקרוי virtual HW interface, היא מקבלת את האשליה שיש לה גישה ישירה לחומרה, אבל בעצם היא לא רצה ב privileged mode ואין לה גישה ישירה לחומרה, אלא ההיפרויזור יוצר לה סביבה מדומה של החומרה לה היא מצפה.
OS1
OS2
∙∙∙
Virtual HW interface
VMM/Hypervisor
Virtualization
Real HW interface
HW (disk, NIC,…)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

13. Type 1 and type 2 hypervisors
וירטואליזציה מלאה נחלקת לשני סוגים. היפרויזור מסוג 1 (bare metal hypervisor) רץ ישירות מעל החומרה, הוא בעצם מעין מערכת הפעלה מצומצמת - microkernel, בעוד היפרויזור מסוג 2 רץ מעל host OS. להיפרויזור מסוג 1 יש יתרונות ברורים מבחינת יעילות ומבחינה זו שאין צורך לשנות את מערכת ההפעלה ואין אפילו צורך בגישה ל-source code. היפרויזור מסוג 2 פשוט יותר להריץ על מחשב שמשמש גם למטרות אחרות (פיתוח, למשל) ולא רק כhost platform. (הוא יכול גם להשתמש בכל השירותים של מערכת ההפעלה, ולכן יכול ככלל לתמוך ביותר HW drivers מאשר היפרויזורים מסוג 1.)
VMware ESX, Microsoft Hyper-V, Xen
VMware Workstation, Microsoft Virtual PC, Sun VirtualBox, QEMU, KVM
Figure 7-1. Location of type 1 and type 2 hypervisors.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

14. Type 1 and type 2 hypervisors (continued)
Figure 7-2. Examples of the various combinations of virtualization type and hypervisor. Type 1 hypervisors always run on the bare metal whereas type 2 hypervisors use the services of an existing host operating system.
VMWare fusion הוא היפרויזור למערכות שמריצות מעה"פ OS X (כמו Mac) עם מעבד אינטל. הוא מאפשר להם להריץ מערכות הפעלה כמו Windows.
KVM – תשתית וירטואליזציה בתוך Linux.
Parallels – וירטואליזציה למחשבי Mac עם מעבדי אינטל.
Wine – open source שמאפשר לאפליקציות ווינדוס לרוץ על מערכות Unix.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

15. What's required of a (classic) hypervisor
Hypervisor should provide the following:
Safety: have full control of virtualized resources
Fidelity: program behavior on VM should be identical to its behavior on bare hardware
Efficiency: As much as possible, run directly on hardware without hypervisor intervention
Full interpretation isn't efficient
מה אנחנו דורשים מתוכנת היפרויזור? ראשית, צריכה להיות לה שליטה מלאה על משאבי המערכת להם נעשית וירטואליזציה – מעבדים, זכרון,
דיסקים, I/O וכולי, מה שאומר שהשליטה הזאת צריכה להילקח מן ה-guest operating system.
Fidelity – משמעותו שהקוד שרץ ב-virtual machine לא יכול כלל להבחין שאינו רץ ישירות על החומרה ומתנהג לכן בדיוק באותה דרך בה היה מתנהג לו רץ ישירות על החומרה.
יעילות – רוב הקוד של ה-VM צריך לרוץ כמות שהוא – natively, כך שהביצועים קרובים לאלו שהיו מתקבלים בריצה ישירות מעל החומרה.
Operating Systems, 2017, Danny Hendler & Amnon Meisels

16. Classic virtualization: trap and emulateHW
VMM
VM1
VM2
Trap (1)
Interrupt handler (2)
HW emulation
Return to process (3)
A classical VMM executes guest operating systems directly, but
at a reduced privilege level (level 1). The VMM (runs in level 0) intercepts traps from the
de-privileged guest, and emulates the trapping instruction against
the virtual machine state. This technique has been extensively described in the literature and it is easily verified
that the resulting VMM meets the Popek and Goldberg criteria.
Emulation is the process of implementing the functionality/interface of one system on a system having different functionality/interface

17. Trap and emulate: difficulties
Sensitive instructions: behave differently in kernel/supervisor and in user mode
I/O instructions, enable/disable interrupts, …
Privileged instructions: cause a trap if executed in user mode
Theorem [Popek and Goldberg, 1974]
A machine can be virtualized [using trap and emulate] if every sensitive instruction is privileged.
It was wrongly assumed that the Popek & Goldberg theorem implies that x86 cannot be virtualized, while in essence, it only implies that it could not be virtualized using trap and emulate. Indeed, VMWare were able to virtualize x86 using binary translation, as we soon see.
Not supported by x86 processors prior to 2005
In 2005, Intel/AMD introduced virtualization HW support
Operating Systems, 2017, Danny Hendler & Amnon Meisels

18. Operating Systems, 2017, Danny Hendler & Amnon Meisels
What is sensitive?
CPU – registers
MMU
Page table
Segments
Interrupts
Timers
I/O devices
Operating Systems, 2017, Danny Hendler & Amnon Meisels

19. X86 virtualization problem I
The x86 architecture (w/o virtualization extensions) cannot be virtualized by trap and emulate
Some sensitive instructions are not privileged
Example: the POPF instruction
Pops 16 bits from stack to the flags register
One of the flags masks (i.e. disables) interrupts
The instruction is not privileged
What happens if the OS of a VM runs POPF ?
Operating Systems, 2017, Danny Hendler & Amnon Meisels

20. X86 virtualization problem II
Some instructions: push, pop, mov can have code segment selectors (cs, ds, ss) as arguments even in user mode, so they can be read
The selectors have two bits that are their current privilege level
x86 (beginning with 386) has four privilege levels (ring 0 to ring 3)
The two lower bits of the cs register are the Current Privilege Level (CPL) of the code
Guest OS thinks that it is in ring 0
Guest OS is actually in a higher ring
Result - guest OS confusion
Operating Systems, 2017, Danny Hendler & Amnon Meisels

21. Implementation options
Emulation –
Full emulation – hypervisor executes code of VM step by step, testing each instruction – prohibitive overhead
Trap and emulate if sensitive instructions  privileged instructions
Change sensitive instructions
Interpretation – equivalent to emulation (BOCHS, JSLinux)
Binary translation – change code on the fly (VMware, QEMU)
Para-virtualization – re-compile guest OS (XEN, Denali)
Hardware assistance – Intel VT-x and AMD-V (used by KVM, XEN, Vmware)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

22. Outline Concepts, classical CPU virtualization Memory virtualization
Binary interpretation
Memory virtualization

23. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Binary translation
Binary translation is the process of translating one instruction set to another one
Approach I: translate statically all code base
In our case the result is para-virtualization
Problems
Dynamically linked libraries are not known at compile time
Self-modifying code, e.g. program generating code and running it, is not covered
Operating Systems, 2017, Danny Hendler & Amnon Meisels

24. Dynamic binary translation
Approach II: translate code on the fly (Just In Time)
Simplest approach
Keep table mapping old instructions to new instructions
Fetch old instruction
Use table to translate
Execute new instruction(s)
Problem: performance
Overhead for every instruction similarly to interpretation
Operating Systems, 2017, Danny Hendler & Amnon Meisels

25. Dynamic BT with caching
Cache translated code region:
After translation run from cache
Translation occurs only once
Static translation cannot handle dynamic control transfer, when:
Jump depending on memory address
Indirect function call (by function pointer)
Translation of dynamic control transfer must be done at execution time
Operating Systems, 2017, Danny Hendler & Amnon Meisels

26. Virtualization prior to HW support
Before HW support was introduced in 2005, VMWare leveraged Intel processor’s protection ring and binary translation. The VMM was put in ring 3 and the OS in ring 1. User code stayed in ring 0. Thus, any access of user code to kernel address space caused a trap. On the other hand, any privileged instruction performed by the kernel trapped to the VMM. Any such trap yielded check, possible emulation, and then binary translation of the next kernel basic block (+ adding a trap at its end – DH), removing sensitive instructions (if any) and replacing them with a trap to the hypervisor. The branch at the end of the basic block is also replaced by a trap to the hypervisor. User process code can mostly be ignored. Translated blocks are cached, so they don’t need to be translated again and again.
Possibly optimization – if a basic block A ends with a jump to basic block B and both were translated, B can be called directly from A without trapping to the hypervisor.
Figure 7-4. Binary translation rewrites the guest operating system running in ring 1, while the hypervisor runs in ring 0

27. VMWare binary translation: example
C code
64-bit binary
Invoking isPrime(49), logging all code translated
Sensitive instructions are rare even in kernels, so performance mainly depends on the translation of regular instructions. Let’s see an example of how regular code is translated.
Above on the left, the C code of a primality test function. Above on the right, resulting x86 64-bit assmbly code, below – binary (hex) representation of the code. THIS is the input to the binary translator. We’ll see an example of what happens when we run this program with input 49.
Translation is done dynamically, in runtime, interleaved with the execution of translated code.
It is done on demand, only if code is executed.
Translation input is the full x86 instruction set (including all sensitive/privileged instructions), output is a safe subset
Binary (hex) representation
Operating Systems, 2017, Danny Hendler & Amnon Meisels

28. VMWare binary translation: example
Translator reads guest memory at the address indicated by guest PC
Decodes instructions, creates Intermediate Representation - IR objects
Accumulates IR objects to translation units (TUs)
Basic blocks (BB), stops upon control flow
First TU
Compiled code fragment (CCF)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

29. VMWare binary translation: example
Translator reads guest memory at the address indicated by guest PC
Decodes instructions, creates Intermediate Representation - IR objects
Accumulates IR objects to translation units (TUs)
Basic blocks (BB), stops upon control flow
Identical code
First TU
Compiled code fragment (CCF)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

30. VMWare binary translation: example
Translator reads guest memory at the address indicated by guest PC
Parses instructions, creates Intermediate Representation - IR objects
Accumulates IR objects to translation units (TUs)
Basic blocks (BB), stops upon control flow
Translation of jump BB
First TU
Compiled code fragment (CCF)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

31. VMWare binary translation: example
Translator reads guest memory at the address indicated by guest PC
Parses instructions, creates Intermediate Representation - IR objects
Accumulates IR objects to translation units (TUs)
Basic blocks (BB), stops upon control flow
Translation of fall through BB
First TU
Compiled code fragment (CCF)
Operating Systems, 2017, Danny Hendler & Amnon Meisels

32. VMWare binary translation: example
C code
64-bit binary
Invoking isPrime(49), logging all code translated
Which basic block will be translated next?
Operating Systems, 2017, Danny Hendler & Amnon Meisels

33. VMWare binary translation: example
C code
64-bit binary
Invoking isPrime(49), logging all code translated
Which basic block will be translated next?
Operating Systems, 2017, Danny Hendler & Amnon Meisels

34. VMWare binary translation: example
C code
64-bit binary
Invoking isPrime(49), logging all code translated
Operating Systems, 2017, Danny Hendler & Amnon Meisels

35. VMWare binary translation example: output

36. VMWare binary translation example: output
These continuations remain because respective basic blocks were not executed

37. VMWare binary translation operation
Translation cache (TC) stores translations done so far
A hash table tracks the input to output correspondence
Chaining optimization allows one CCF to jump directly to another without calling out of the translation cache
As TC gradually captures guest's working set, proportion of translation decreases
User code does not have to be translated
Operating Systems, 2017, Danny Hendler & Amnon Meisels

38. Dealing with privileged instructions: example
The cli (clear interrupts) instruction is privileged
Translated to: “vcpu.flags.IP=0”
Much faster than source binary!
Operating Systems, 2017, Danny Hendler & Amnon Meisels

39. Outline Concepts, classical CPU virtualization Memory virtualization
Binary interpretation
Memory virtualization

40. Memory allocation
Each VM usually receives a contiguous set of physical addresses.
512 Mbyte – 4 Gbyte are typical values
As far as VM is concerned, this is the physical memory of the machine
The guest OS allocates pages or segments to guest processes

41. Memory management VM OS must not manage real page table
Assumptions of OS in VM:
Physical memory is a contiguous block of addresses from 0 to some n
OS can map any virtual page to any page frame
Hypervisor must:
Partition memory among VMs
Ensure virtual page mapping only to assigned page frames
TLB – page fault in HW-managed TLB (e.g. x86) causes HW to select a page from page table
VM OS must not manage real page table

42. Option 1: brute force Guest OS Hypervisor TLB CPU CR3 HW
Define these pages as not R/W
Guest OS
Hypervisor
Page table
VMM SW
VM memory layout
Page dir.
Interrupt & VMM corrects address.
TLB
CPU
CR3 HW

43. Brute force – description
Guest page tables are read and write protected in host system.
If guest OS reads page table (e.g. for page eviction) writes page table (e.g. after page fault), or changes CR3, the system traps.
The hypervisor then uses a VM memory layout to:
Return answers to VM
Update the layout
Hypervisor switches VM memory layout when new VM is scheduled.

44. Option 2: shadow page tables
Guest OS
Hypervisor
Page table
VMM SW
Shadow page table
Page dir.
Interrupt & VMM corrects page table.
G-CR3
TLB
CPU
CR3 HW

45. Shadow page tables – description
Hypervisor maintains “shadow page tables”.
Guest page tables map: Guest VA Guest PA
Shadow tables: Guest VA Host PA.
Hypervisor does not trap guest updates to its page table.
Result – inconsistent guest page table and shadow page table.
When guest process accesses virtual address
The physical address is not in the guest page table, but in the shadow page table.
HW translates correctly, because it is aware only of shadow tables.

46. Shadow page tables – description (continued)
If address in TLB – TLB hit and no problem.
When guest process causes a page fault
Hypervisor begins execution.
Hypervisor updates guest page table with new page.
Hypervisor updates shadow page table.
Performance is as good as native execution as long as there are no page faults.
Shadow page tables should be cached so that once a VM is re-scheduled the page table does not have to be rebuilt from scratch.

47. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Shadow page tables – page faults (continued)
Two scenarios when handling a page fault. Hypervisor ``walks’’ guest page table to determine which it is.
Guest page fault – No translation in guest page tables  ``inject’’ page fault for guest to handle
Guest translation found  update shadow table respectively
Operating Systems, 2017, Danny Hendler & Amnon Meisels

48. Shadow page tables – updating CR3
Guest
Page Table
Guest
Page Table
Guest
Page Table
Virtual CR3
Real CR3
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Whenever the guest OS changes CR3 (for instance, upon a process context switch), it traps to the VMM which changes the shadow CR3 corresponding to the respective VM accordingly.
Slide taken from a presentation by VMWare.

49. Shadow page tables – updating CR3
Guest
Page Table
Guest
Page Table
Guest
Page Table
Virtual CR3
Real CR3
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Slide taken from a presentation by VMWare.

50. Shadow page tables – updating CR3
Guest
Page Table
Guest
Page Table
Guest
Page Table
Virtual CR3
Real CR3
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Slide taken from a presentation by VMWare.

51. Undiscovered guest page table
Virtual CR3
Real CR3
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Whenever the VMM encounters a new value of the CR3 (pointing to a new page table), it should create a new instance of the shadow page tables structure.
Slide taken from a presentation by VMWare.

52. Undiscovered guest page table
Virtual CR3
Real CR3
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Shadow
Page Table
Slide taken from a presentation by VMWare.

53. Option 3: nested page tables
Guest OS
Hypervisor
Page table
VMM SW
Host page table
Page dir.
TLB
CPU
CR3
EPTP HW

54. Nested page tables - description
The name implies having page tables within page tables.
The essence of the idea is a hardware assist.
Hardware has an extra pointer and the ability to walk an extra set of page tables.
Idea is called Extended Page Tables (EPT) by Intel
Guest page tables hold Guest VA Guest PA mapping, access by standard CR3
Extended page tables hold Host VA  Host PA mapping, access by EPTP (EPT pointer).
Host VA=Guest PA

55. Walking Nested page tables

56. Nested page tables – description (cont'd)
TLB as usual holds Guest VA Host PA
On memory access
If found in TLB – no problem.
If not in TLB, but no page fault, hardware walks both tables and updates TLB.
If page fault, then hardware hypervisor gets host physical page and provides host virtual page (guest physical) to VM.

57. Operating Systems, 2017, Danny Hendler & Amnon Meisels
Sources
“Modern operating systems”, 4‘th edition, A. Tanenbaum and H. Bos
“Virtual machines”, J. E. Smith and R. Nair
A presentation by Niv Gilboa from
“Formal requirements for virtualizable third generation architectures”, G. J. Popek and R. P. Goldberg, CACM, 1974
“A comparison of software and hardware techniques for x86 virtualization”, K. Adams and O. Ageson, ASPLOS 2006
A presentation by VMWare
CR3 – x86 register pointing to the page table hierarchy
Operating Systems, 2017, Danny Hendler & Amnon Meisels