Lecture 5. User-Mode Linux. Jeff Dike. November 7, Operating Systems Practical. OSP Lecture 5, UML 1/33

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1 Lecture 5 User-Mode Linux Jeff Dike Operating Systems Practical November 7, 2012 OSP Lecture 5, UML 1/33

2 Contents User-Mode Linux Keywords Resources Questions OSP Lecture 5, UML 2/33

3 Outline User-Mode Linux Keywords Resources Questions OSP Lecture 5, UML 3/33

4 UML A linux kernel port on Linux A virtual machine in user-space The simulated hardware is built on top of the native kernel services The UML kernel is ported on top of the native s kernel system calls The associated code is in the arch interface (arch/um/) There are no drivers Processes run in a closed environment OSP Lecture 5, UML 4/33

5 UML (2) OSP Lecture 5, UML 5/33

6 Devices console the main console is the one in which the UML kernel was started subsequent consoles run inside an xterm block devices emulated through files serial links emulated through pseudo-terminals (/dev/pts/0) networking daemon used to send Ethernet frames between virtual machines can link the virtual device to the real one OSP Lecture 5, UML 6/33

7 Processor implemented using the arch interface the entire code is a separate architecture named um : # l s / u s r / s r c / l i n u x / a r c h /um/ K c o n f i g K c o n f i g. x86 64 M a k e f i l e s k a s [... ] K c o n f i g. c h a r M a k e f i l e M a k e f i l e t t [... ] K c o n f i g. debug M a k e f i l e i M a k e f i l e x86 64 os L i n u x K c o n f i g. i M a k e f i l e i a 6 4 c o n f i g. r e l e a s e s c r i p t s K c o n f i g. n e t M a k e f i l e os L i n u x d e f c o n f i g sys i K c o n f i g. s c s i M a k e f i l e ppc d r i v e r s sys i a 6 4 user space code need to be able to run unmodified in the virtual machine system calls are interpreted and run on the virtual machine UML runs in user space as a process OSP Lecture 5, UML 7/33

8 Intercepting system calls uses ptrace - controlled execution (gdp uses ptrace) one thread uses ptrace to control the other threads and processes the thread is notified by a system call from another thread the arguments are gathered redirects to kernel code running in user space for execution OSP Lecture 5, UML 8/33

9 Trap traps are used to switch from user mode to kernel mode on physical systems are generated by hardware components the result is forcing the processor to jump to a certain address in kernel space traps are implemented using Linux signals SIGALRM/SIGVTALRM - clock SIGIO - hardware interrupts SIGSEGV - memory faults the user-space kernel declares handlers for these signals OSP Lecture 5, UML 9/33

10 Trap (2) signal handlers need to run in kernel-mode (in the UML process) 1. need to use a kernel stack 2. need to deactivate the interception of system calls OSP Lecture 5, UML 10/33

11 Trap (3) OSP Lecture 5, UML 11/33

12 Kernel address space when a process enters kernel mode, it automatically changes the address space the UML problem? the kernel and the process co-exist in the same address space - the UML process address space solution: placing the UML kernel in a memory area that is not likely to be accessed (0xa xbfffffff) mmaps a file in each process space OSP Lecture 5, UML 12/33

13 Process address space Each process on the virtual machine has a real process on the physical machine associated with it All processes share kernel data mmap a file with kernel data in the address space of each andevery process (shared segment) context changes are implemented using real (native) context changes what preempts a process? clock interrupt on the native kernel SIGVTALRM on the UML kernel OSP Lecture 5, UML 13/33

14 Address space OSP Lecture 5, UML 14/33

15 Initialization and shutdown example:./linux rc5 ubda=fedoracore5-x86-root fs mem=128m arguments are sent through a buffer init memory, start idle thread the monitoring thread starts intercepting start kernel, mem init, paging init register and init drivers on shutdown, all processes and threads are killed OSP Lecture 5, UML 15/33

16 Processes for creating a new process, the generic code in the kernle calls the specific architecture code from /arch/ in the UML case, a new process is created on the host system the monitoring thread is used (MT) new process/thread executes init operation (handlers for SIGVTALRM, SIGIO, SIGSEGV, etc.) after init it sends itself a SIGSTOP the MT detects the stopping of the process and ends the system call and returns a return value specific to fork the process is killed on the host system and the memory is freed OSP Lecture 5, UML 16/33

17 System calls The virtualization of the system calls is done through MT system calls are redirected to the virtual kernel the system call is mapped to a getpid on the host system OSP Lecture 5, UML 17/33

18 System calls (2) how to call the system call switch on the kernel stack? 1. creating an execution context that positions the process at the beginning of the switch statement 2. use a signal at the return from the kernel; the handler for this signal is the execution of the system call switch statement the MT is notified at the end of the system call in the MT the MT stores the return value in the specialized register the process continues to execute user level code OSP Lecture 5, UML 18/33

19 System calls (3) OSP Lecture 5, UML 19/33

20 System calls (4) OSP Lecture 5, UML 20/33

21 Context switch in the case of a process switch - a process calls schedule a new process is chosen and the architecture dependent code is called the MT is notified from the arch/um/ code the MT stops the process and starts the new one OSP Lecture 5, UML 21/33

22 Context switch (2) after re-planning, some pages can be swapped - but the mapping exists the pages are stored in a circular buffer after re-planning this buffer is checked and the address space is updated OSP Lecture 5, UML 22/33

23 Delivering signals delivered signals are stored in a queue in the process task struct the queue is inspected upon every exit from kernel mode the signal is delivered to the process running on the host kernel through SIGUSR2 the SIGUSR2 handler executes the actual signal handler OSP Lecture 5, UML 23/33

24 Memory faults what is demand paging? a memory fault causes the delivery of SIGSEGV to the UML process the handler checks the nature of the fault: user-mode fault or kernel-mode fault if the page is valid - it is mapped otherwise SIGSEGV is sent to the user process or kernel panic exception: sending of a invalid pointer from user-space or kernel space checks the address of the instruction that generated the fault OSP Lecture 5, UML 24/33

25 IRQ copied from i386 for i386: the interrupt routine is called through do irq for um: interrupts are simulated through SIGIO the routine is chosen through the file descriptor that is associated with the device OSP Lecture 5, UML 25/33

26 The result a Linux virtual machine runs on a Linux host operating system native applications run un-modified on UML has the advantage of using the latest kernel (over other virtualization techniques) from 2.6 up, the um architecture is included in the kernel from 2.6 up SKAS (separate kernel address space) is used instead of MT OSP Lecture 5, UML 26/33

27 Applications kernel debugging isolation prototiping (testing on a virtual system before launching on the physical system) multiple environments on the same physical system OSP Lecture 5, UML 27/33

28 Outline User-Mode Linux Keywords Resources Questions OSP Lecture 5, UML 28/33

29 Keywords Linux kernel user-mode context switch address space system calls traps memory faults IRQ OSP Lecture 5, UML 29/33

30 Outline User-Mode Linux Keywords Resources Questions OSP Lecture 5, UML 30/33

31 Resources proceedings/als00/2000papers/papers/full_papers/ dike/ proceedings/als01/full_papers/dike/ UserModeLinux-HOWTO.html pool_h01/ OSP Lecture 5, UML 31/33

32 Outline User-Mode Linux Keywords Resources Questions OSP Lecture 5, UML 32/33

33 Questions? OSP Lecture 5, UML 33/33

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