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CSE 153 Design of Operating Systems Fall 2016 Lecture 15: Virtual Machine Monitors OS Abstractions Applications Process I/O Devices File system Virtual memory Network Operating System CPU Hardware Disk RAM CSE 153 – Lecture 16 – Virtual Machine Monitors 2 Virtual Machine Monitor Applications Process I/O Devices File system Virtual memory Network Operating System Hardware Virtual Machine Monitor CPU Disk RAM CSE 153 – Lecture 16 – Virtual Machine Monitors 3 What is a VMM? We have seen that an OS already virtualizes Syscalls, processes, virtual memory, file system, etc. Applications program to this interface OS offers illusion that each process is running in isolation A VMM virtualizes an entire physical machine Interface supported is the hardware VMM offers illusion that OS has full control over the hardware VMM “applications” (OSes) run in virtual machines Implications of booting an OS in a virtual machine Run multiple instances of an OS on same physical machine Run different OSes simultaneously on the same machine » Linux on Windows, Windows on Mac, etc. CSE 153 – Lecture 16 – Virtual Machine Monitors 5 Why do such a crazy thing? Resource utilization Machines today are powerful, want to multiplex their hardware » e.g., Cloud service can divvy up a physical machine to customers Can migrate VMs from one machine to another without shutdown Software use and development Can run multiple OSes simultaneously » No need to dual boot Can do system (e.g., OS) development at user-level » Xv6/Qemu! Strong isolation – for security and otherwise Many other cool applications Debugging, emulation, security, speculation, fault tolerance… Applications/services at the granularity of a machine Specific version of OS, libraries, applications, etc., as package CSE 153 – Lecture 16 – Virtual Machine Monitors 6 Example of Cool VMM Tricks Time dilation VMM can control the rate of timer interrupts to OS Can change how OS interprets passage of time If VMM slows timer by 10x, then other hardware (CPU, disk, network) appears 10x faster to OS and applications » OS reads 100 MB of data from network in 1 second, but thinks only 0.1 second has elapsed, giving illusion of 1 GBps of bandwidth Can experiment with apps, protocols, and systems on future hardware » But, applications run 10x slower CSE 153 – Lecture 16 – Virtual Machine Monitors 7 VMM Requirements Fidelity OSes and applications work the same without modification » (although we may modify the OS a bit) Isolation VMM protects resources and VMs from each other Performance VMM is another layer of software…and therefore overhead » As with OS, want to minimize this overhead VMware: » CPU-intensive apps: 2-10% overhead » I/O-intensive apps: 25-60% overhead CSE 153 – Lecture 16 – Virtual Machine Monitors 8 VMware VMware workstation uses hosted model VMM runs unprivileged, installed on base OS Relies upon base OS for device functionality VMware ESX server uses hypervisor model VMM runs directly on hardware VMware uses software virtualization Dynamic binary rewriting translates code executed in VM » Rewrite privileged instructions with emulation code (may trap) CPU only executes translated code Think JIT compilation for JVM, but » full binary x86 IR code safe subset of x86 Incurs overhead, but can be well-tuned (small % hit) CSE 153 – Lecture 16 – Virtual Machine Monitors 11 Vmware Hypervisor Model Applications Operating System Hardware Virtual Machine Monitor Hardware CSE 153 – Lecture 16 – Virtual Machine Monitors 12 VMware Hosted Architecture CSE 153 – Lecture 16 – Virtual Machine Monitors 13 Xen Early versions use “paravirtualization” Fancy word for “we have to modify & recompile the OS” Since you’re modifying the OS, make life easy for yourself Create a VMM interface to minimize porting and overhead Xen hypervisor (VMM) implements interface VMM runs at privilege, VMs (domains) run unprivileged Trusted OS (Linux) runs in own domain (Domain0) » Use Domain0 to manage system, operate devices, etc. Most recent version of Xen does not require OS mods Because of Intel/AMD hardware support Commercialized via XenSource, but also open source CSE 153 – Lecture 16 – Virtual Machine Monitors 14 Xen Architecture CSE 153 – Lecture 16 – Virtual Machine Monitors 15 What needs to be virtualized? Exactly what you would expect CPU Events Memory I/O devices Isn’t this just duplicating OS functionality in a VMM? Yes and no Approaches will be similar to what we do with OSes » Simpler in functionality, though (VMM much smaller than OS) But implements a different abstraction » Hardware interface vs. OS interface CSE 153 – Lecture 16 – Virtual Machine Monitors 17 Virtualizing Privileged Insts OSes can no longer successfully execute privileged instructions Virtual memory registers, interrupts, I/O, halt, etc. For those instructions that cause an exception Trap to VMM, take care of business, return to OS in VM For those that do not… Xen: modify OS to hypervisor call into VMM VMware: rewrite OS instructions to emulate or call into VMM CSE 153 – Lecture 16 – Virtual Machine Monitors 18 Virtualizing Memory OSes assume they have full control over memory Managing it: OS assumes it owns it all Mapping it: OS assumes it can map any virtual page to any physical page But VMM partitions memory among VMs VMM needs to assign hardware pages to VMs VMM needs to control mappings for isolation » Cannot allow an OS to map a virtual page to any hardware page » OS can only map to a hardware page given to it by the VMM Hardware-managed TLBs make this difficult When the TLB misses, the hardware automatically walks the page tables in memory As a result, VMM needs to control access by OS to page tables CSE 153 – Lecture 16 – Virtual Machine Monitors 22 Shadow Page Tables Three abstractions of memory Machine: actual hardware memory » 2 GB of DRAM Physical: abstraction of hardware memory managed by OS » If a VMM allocates 512 MB to a VM, the OS thinks the computer has 512 MB of contiguous physical memory » (Underlying machine memory may be discontiguous) Virtual: virtual address space per process » Standard 232 address space In each VM, OS creates and manages page tables for its virtual address spaces without modification But these page tables are not used by the MMU hardware CSE 153 – Lecture 16 – Virtual Machine Monitors 24 Shadow Page Tables CSE 153 – Lecture 16 – Virtual Machine Monitors 26 Smartphones == CSE 153 – Lecture 16 – Virtual Machine Monitors 31 …in 2014 CSE 153 – Lecture 16 – Virtual Machine Monitors 32 MALWARE Android has become a hot target of malware authors It is not a problem of the platform itself Data from McAfee CSE 153 – Lecture 16 – Virtual Machine Monitors 34 Android CSE 153 – Lecture 16 – Virtual Machine Monitors 36 Android CSE 153 – Lecture 16 – Virtual Machine Monitors 39 Android CSE 153 – Lecture 16 – Virtual Machine Monitors 40 Android CSE 153 – Lecture 16 – Virtual Machine Monitors 41 Dalvik VM Dalvik is a Java Virtual Machine Designed to run with limited system resources Android applications are written in Java Every application runs in its own JVM CSE 153 – Lecture 16 – Virtual Machine Monitors 42 DEX File Java class files are converted into “.dex” files that Dalvik executes Java byte-code is converted into Dalvik bytecode during this process CSE 153 – Lecture 16 – Virtual Machine Monitors 43 Process management What’s the difference between mobile app cycle and desktop app cycle? One app has focus from the user at a time Two key principles Android usually does not delete an app’s state even if you close it Android kills apps when the memory usage goes too high, but it saves app state for quick restart later on CSE 153 – Lecture 16 – Virtual Machine Monitors 45 Application Life cycle Start Run Paus e End CSE 153 – Lecture 16 – Virtual Machine Monitors 46 Example System Home Home At the “Home” screen CSE 153 – Lecture 16 – Virtual Machine Monitors 48 Example System Home Mail Home List Start the “Mail” app and read the list CSE 153 – Lecture 16 – Virtual Machine Monitors 49 Example System Home Mail Home List Message Click on one of the message and see its content CSE 153 – Lecture 16 – Virtual Machine Monitors 50 Example System Home Mail Browser Home List Message Browser Click a link in the message CSE 153 – Lecture 16 – Virtual Machine Monitors 51 Example System Home Browser Home Browser Now we have enough space to start the “Map” app March 4, 2014 CSE 153 – Lecture 16 – Virtual Machine Monitors 52 Example System Home Browser Home Browser Map Map Start the “Map” app CSE 153 – Lecture 16 – Virtual Machine Monitors 53 Example System Home Browser Home Browser Map Go back to the browser CSE 153 – Lecture 16 – Virtual Machine Monitors 54 Example System Home Home Browser Mail List Message “Mail” app is resumed and shows previous message CSE 153 – Lecture 16 – Virtual Machine Monitors 55 Example System Home Home Browser Mail List Go back to the mail list CSE 153 – Lecture 16 – Virtual Machine Monitors 56 Example System Home Home Browser Mail Go back to the “Home” screen CSE 153 – Lecture 16 – Virtual Machine Monitors 57 Memory management Linux kernel does most of the job Page-based memory management Virtual address to physical address mapping No virtual memory swapping Pros and cons? CSE 153 – Lecture 16 – Virtual Machine Monitors 58 Offloading of computation Naive offloading Speech-to-text, OCR More sophisticated offloading - fine-grained offloading MAUI: Making Smartphones Last Longer with Code Offload Running two versions of the app on the mobile device and a powerful server Decide when/what to offload on the fly CSE 153 – Lecture 16 – Virtual Machine Monitors 60 Disk I/O Flash Hard Disk Drive Random access ~0.1ms 5-10ms File fragment impact No Greatly impacted Total power 1/2 to 1/3 of HDD up to 15+ watts Reliability Reliable Less reliable due to mechanical parts Write longevity Limited number of writes Less of a problem Capacity <= 512GB 2-3TB Price $1.5-2 / GB $0.1-0.2 / GB CSE 153 – Lecture 16 – Virtual Machine Monitors 61 Summary VMMs multiplex virtual machines on hardware Export the hardware interface Run OSes in VMs, apps in OSes unmodified Run different versions, kinds of OSes simultaneously Android: Use of virtual machines to protect phones CSE 153 – Lecture 16 – Virtual Machine Monitors 63 Other VMM Systems VMWare Xen Microsoft Virtual PC User-Mode Linux CSE 153 – Lecture 16 – Virtual Machine Monitors 64
