CS 550 Operating Systems Spring Introduction to Virtual Machines

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1 CS 550 Operating Systems Spring 2018 Introduction to Virtual Machines 1

2 How to share a physical computer Operating systems allows multiple processes/applications to run simultaneously Via process/memory virtualization, CPU scheduler, multithreading, concurrency control Allow multiple users to access the same machines APP1 APP2 APP3 APP4 Operating System 2

3 Problems? All users and applications have to share a single operating system (the same kernel). Can we have multiple operating systems but still on the same physical machine? APP1 APP2 APP3 APP4 Operating System 3

4 A Straightforward Solution To create processes/threads to emulate different types of operating systems. Each instance is usually called a virtual machine Applications can run inside these virtual machines VM1 VM2 VM3 VM4 APP1 APP2 APP3 APP4 OS1 OS2 OS3 OS4 Host Operating System 4

5 Virtualization Definition A technique for hiding the physical characteristics of computing resources from the way in which other systems, applications, or end users interact with those resources. Make a single physical resource appear to function as multiple logical resources 5

6 Old idea from the 1960s IBM VM/370 A VMM for IBM mainframe Multiple OS environments on expensive hardware Desirable when few machine around Popular research idea in 1960s and 1970s Entire conferences on virtual machine monitors Hardware/VMM/OS designed together Interest died out in the 1980s and 1990s Hardware got more cheaper Operating systems got more powerful (e.g. multi-user) 6

7 A Return to Virtual Machines Commercial virtual machines for x86 architecture VMware (1999-) Connectix Virtual PC (now Microsoft) KVM for Linux Research virtual machines for x86 architecture Xen (SOSP 03) plex86 7

8 Why Return? Single core becomes more powerful More and more cores per CPU Overall CPU% is low (in data centers) 15% ~ 18% How to efficiently use them? -- Sharing 8

9 Virtual Machines Run multiple (virtual)machines on the same server to share the same physical resources, including CPU, memory, storage, etc. Provide the illusion to the virtual machines that they exclusively occupy the physical resources 9

10 How to Build Virtual Machines What we have: We have guest operating system source code, or executable Binary code (such as Linux and Windows) Or we have the OS program Why don t we load it to the memory and create processes? Host (Hypervisor) Guest (VM) 10

11 Recall: CPU Fetch and Execution Model Instruction Guest (VM) Instruction Instruction Instruction CPU Data Memory 11

12 Unfortunately, Some of the instructions cannot be executed by the Guest OS anymore Privilege instructions: Modify interrupt flags Alter segmentation Affect protection mechanism Perform peripheral I/O Host (Hypervisor) Guest (VM) Physical CPU R 12

13 Solution For example, x86 processor s segment-protection mechanism recognizes 4 privilege levels (0-high, 3-low level) Place Guest OS in Ring 1, and VMM in Ring 0 (with the highest privilege) Ring 3 User Apps Ring 2 Ring 1 Ring 0 Guest OS Hypervisor (VMM) Host Computer

14 Solution cont, Ring 3 User Apps What happens, if Guest OS executes privilege instructions? Cause a trap if executed in privilege level other than Ring 0 Then host (VMM/hypervisor) can interpret such instructions And do something Ring 2 Ring 1 Ring 0 Guest OS Hypervisor (VMM) Host Computer

15 How About Memory Virtualization Dom U Dom U Dom U Page Table Page Table Page Table Page Table Host/VMM Host Computer

16 Two Page Tables VM VMM Machine address space page belonging to VM1

17 Software MMU GVA - > GFN -> PFN GVA - > GFN GFN - > PFN GVA -> PFN How can Shadow page table captures the guest page table updates? 17

18 Hardware MMU Using EPT, the guest operating system continues to maintain GVA->GFN mappings in the GUEST page tables VMM maintains GFN->PFN mappings in the HOST page tables Both the GUEST page tables and the HOST page tables are exposed to the hardware When a logical address is accessed, the hardware walks the guest page tables (GVA- >GFN) and also walks the HOST page tables (GFN->PFN) to determine the corresponding (GVA->PFN) 18

19 How About I/O Device Virtualization No Modifications to Guest VM trap and emulation Using emulator: QEMU is a userspace process which emulates various types of I/O devices Guest QEMU VMEXITS Hypervisor Kernel 19

20 Main Problems of VM Virtualization Performance, Performance and Performance

Virtualization. Pradipta De

Virtualization. Pradipta De Virtualization Pradipta De pradipta.de@sunykorea.ac.kr Today s Topic Virtualization Basics System Virtualization Techniques CSE506: Ext Filesystem 2 Virtualization? A virtual machine (VM) is an emulation