Limited Direct Execution
### CPU Virtualization Recall - A single physical CPU can support the illusion of having multiple processes running concurrently. - The sharing of the CPU among these processes is *time-sharing*.
### Design Goals of CPU Virtualization - Performance - The process can be itself and run as fast as possible without frequent interaction with the OS. - Control - We want to avoid the scenario where a process can run forever and take over all the machine's resources or a process performs unauthorized actions. This requires interaction with the OS.
### How to efficiently virtualize the CPU with control, that is the question. - Efficiently - Control
### Efficient - The most efficient way to execute a process is through direct execution.
### Problem with direct execution - Once the program begins to run, the OS becomes a complete outsider - No control over the running program. - The program can access anything it wants to, including **restricted operations** (direct access to hardware devices, especially I/O for unauthorized purposes - think *sudo for all*). - The program may never **switch** to a different process without explicit. instructions in **main()**, thus defeating the purposes of **time-sharing**.
### Problem with direct execution: Restricted Operations - The process should be able to perform restricted operations, such as disk I/O, open network connections, etc. - But we should **not** give the process **complete control** of the system. - Reiteration: The process should be able to have its restricted operations performed. - Solution: hardware support via processor modes: - User mode - Kernel mode
### Process modes - A **mode bit** is added to hardware to support distinguishing between user mode and kernel mode. - Some instructions are designated as **privileged instructions** that cannot be run in user mode (only in kernel mode). - A user-mode process trying to perform privileged instructions will raise a *protection-=fault* and be killed. - How can these instructions be called by a process in user-mode? - **System call**
### System calls - A **small** set of APIs for restricted operations - [XV6 system calls](https://github.com/mit-pdos/xv6-public/blob/master/syscall.h) - Linux x86_64 has 335 systems called (0 to 334) - Spring 2019 - Linux uses a sys_call_table to keep the syscall handlers. - [syscall_64.tbl](https://github.com/torvalds/linux/blob/6f0d349d922ba44e4348a17a78ea51b7135965b1/arch/x86/entry/syscalls/syscall_64.tbl) - These system calls enable user-mode process to have restricted operations performed without having to gain complete control over the system.
### How does a system call happen? - To make a system call, the process need to switch form user mode to kernel mode, do the privileged operation, and then switch back. - This is done through hardware support - Require assembly instructions - **trap**: go from user mode to kernel mode. - **return-from-trap**: go back from kernel mode to user mode.
### Example: Linux write system call
### System calls versus normal C calls - Function declarations are the same. - System calls - have **trap instruction** in them. - have extra level of indirection (movements between modes). - perform restricted operations - have bigger overhead and are slower than equivalent function calls. - can use kernel stack.
### System call naming: - User space definition: - [libc's definition for *write()*](https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=io/write.c;hb=HEAD) - Kernel definition: - [Linux definition for *write()*](https://github.com/torvalds/linux/blob/74673fc50babc9be22b32c4ce697fceb51c7671a/include/linux/syscalls.h) - The user space definition will eventually call the kernel definition.
### Problem with direct execution: Switching process - A free running process may never stop or switch to another process. - OS needs to control the process, but how? - Once a process is running, OS is NOT running (OS is but another process) - The question: How can OS **regain control** of the CPU from a process so that it can switch to another process?
### First approach: Cooperative processes - The OS trusts the processes to behave cooperatively - A process that runs for too long would voluntarily give up the CPU and hand control to the OS. - This is done by the process making a **yield()** system call periodically. - When process performs an invalid/illegal operation (Exception raised).
### First approach: Cooperative processes - All programmers promise to insert **yield()** into their code to give up CPU resources to other people's program. - We have solved the second problem and achieved eternal world peace. - Even in a perfect world, what happens if a process falls into an infinite loop prior to calling **yield()**? - [Collaborative multitasking (Windows 3.1X, Mac PowerPC)](https://en.wikipedia.org/wiki/Cooperative_multitasking)
### When you run a program from the **prompt**, the shell ... - finds out where the program is in the file system. - calls *fork()* to create a new child process to run the command. - calls one of *exec()* family to run the command. - calls *wait()* to wait for the child process to finish before giving user the **prompt** again.
### Second approach: Non-cooperative Processes - Similar to processor modes, the hardware once again provided assistance via **Timer Interrupt**. - A timer device can be programmed to raise an interrupt periodically. - When the interrupt is raised, the running process stops, a pre-configured interrupt handler in the OS runs. - The OS regains control.
### Second approach: Non-cooperative Processes - The OS first decides which process to switch to (with the help from the **scheduler**) - The OS executes a piece of assemble code (context switch. - Save register values of the **currenlty running** process into its kernel stack. - Restore register values of the **soon running** process from its kernel stack.
### Getting down and dirty We will be working with MIT's XV6 operating system in the future. Perform the following commands in your VM to set up the neccessary infrastructure: ``` $ cd ~ $ sudo apt-get update $ sudo apt-get install -y qemu $ sudo apt-get install -y tmux $ git clone git://github.com/mit-pdos/xv6-public.github $ cd xv6-public $ make $ make qemu-nox ``` To exit from XV6, hit `Ctrl-A` then `X`