Add a system call to xv6

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What is a system call?

If you did follow my previous blog post, you would already have an insight into the xv6 operating system. However, just to brief the new readers, xv6 is a teaching operating system which was developed by MIT. When a program in user mode requires access to RAM or a hardware resource, it must ask the kernel to provide access to that particular resource. When a program makes a system call, the mode is switched from user mode to kernel mode. There are many system calls in an operating system which executes different types of tasks when they are called. In order to define your own system call in xv6, you need to make changes to 5 files. Namely, these files are as follows. We would start the procedure by editing syscall. This file already contains 21 system calls. In order to add the custom system call, the following line needs to be added to this file. Next, we need to add a pointer to the system call in the syscall. This file contains an array of function pointers which uses the above-defined numbers indexes as pointers to system calls which are defined in a different location. In order to add our custom system call, add the following line to this file. Therefore, we need to implement the system call function. However, we do not implement the system call function in the syscall. Instead, we only add the function prototype in here and we define the function implementation in a different file. The function prototype which needs to be added to the syscall. Next, we will implement the system call function. In order to do this, open the sysproc. The basic implementation of the system call is now complete. However, there are 2 more minor steps remaining. In order for a user program to call the system call, an interface needs to be added. Therefore, we need to edit the usys. S file where we should add the following line. This would be the function which the user program calls. If you have completed all of the above, you have successfully added a new system call to xv6.

What is a system call?

GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. If nothing happens, download GitHub Desktop and try again. If nothing happens, download Xcode and try again. If nothing happens, download the GitHub extension for Visual Studio and try again. Here we try to create a basic system-call that can be used when you try to implement your scheduler functions. The first step is to extend the current proc structure and add new fields ctimeetime and rtime for creation time, end-time and total time respectively of a process. When a new process gets created the kernel code should update the process creation time. The run-time should get updated after every clock tick for the process. To extract this information from the kernel add a new system call which extends wait. The new call will be. The two arguments are pointers to integers to which waitx will assign the total number of clock ticks during which process was waiting and total number of clock ticks when the process was running. The return values for waitx should be same as that of wait system-call. Create a test program which utilises the waitx system-call by creating a time like command for the same. Now the stime gets updated when the command is first scheduled on the CPU. End time is called when the program exits. Now that you know how to add system-calls to Xv6, lets extend the idea. Replace the current round robin scheduler for Xv6 and replace it with a priority based scheduler. A priority based scheduler selects the process with highest priority for execution. In case two or more processes have same priority, we choose them in a round robin fashion. The priority of a process can be in the range [0,]smaller value will represent higher priority. Set the default priority of a process as The system-call returns the old-priority value of the process. In case the the priority of the process increases the value is lower than beforethen rescheduling should be done. Make a report which includes a small example and a comparison of your current priority based scheduling policy and round robin approach.

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Your assignment is to download and build the xv6 operating system and boot it on an x86 emulator. You will add a new system call called tracewhich turns console-based system call tracing for the calling process on and off. The system call also reports the total number of system calls called by the process since it started. This assignment entails writing only a miniscule amount of code. Given the large class size, however, you may work on this assignment in a group comprising up to five members. Be sure to indicate the group members clearly in your report. If you have over three members in your group, you must implement the optional part of the assignment: the csinfo system call, which returns the number of context switches that the process has experienced so far. Moreover, it was written with the PDP—11 architecture in mind. It was designed at MIT as a small, easy-to-understand operating system that is suitable for teaching. For information about xv6, take a look at its home page at MIT. A detailed description of the structure of xv6, including its system calls, file system, and memory management can be found in the xv6 book :. There are several ways to get xv6 up and running. It can run natively or through one of several x86 simualtors. QEMU is an open source machine emulator and virtualizer. It allows you to run software for one machine on a different machine architecture. It also makes it easy to run and debug opearating systems since the complete operating system just runs as another process on your system. I tested the process on my Mac but expect it to work on other operating systems. You can check this out for instructions on running xv6 directly on Windows. I have not tried it. The entire xv6 package can be obtained from his posting to github. To install it into a directory called xv6, run:. A patch to the makefile and an extra helper script to pack up the modifications on a particular branch as patches and tar them up for submission. Go directly to step 8. Download VirtualBox. You will be presented with a list of packages. Download the version suitable for your system e. Run VirtualBox and click on New top left icon to create a new virtual machine. Select the following options:. The virtual machine is set up, powered off, and no operating system is yet installed. Download Ubuntu. I downloaded Ubuntu The next page asks you for money. You now downloaded a disk image of the server: ubuntu— Power on the virtual machine by clicking the Start button in the list of top left icons. You can get a terminal from the Ubuntu desktop by pressing ctrl-alt-T.

XV6 System Call

In this first xv6 assignment you will be modifying and adding code to a number of different files that implement the kernel. You won't be adding much source code; rather, the goal is to get you comfortable with the workflow so that future assignments requiring more extensive changes to the codebase won't be too intimidating! You should've received an e-mail invitation to share a private repository with me on BitBucket you need to have accepted this invitation in order to continue. Assuming you have Git installed on your machine, simply run the following command with your own username to clone your repository into a directory named xv6 on your machine. Next, you should create an upstream branch that tracks my public xv6 repository, just in case I push any more commits to it for upcoming machine problems. You can now fetch and merge the master branch from this upstream repository into your own master branch. Nothing should happen, as I won't have pushed any new commits yet. Note: if you prefer to watch a full demonstration of the following instructions first, just scroll down for a video recording. In order to run xv6, we'll be using a combination of the VirtualBox virtualization platform and the Vagrant virtual environment management tool. Download and install them first, then come back here — download links follow:. While VirtualBox comes with a graphical user interface, we won't be using it directly. Instead, everything will be done through Vagrant's command line interface. If everything's been installed properly, this command will download the necessary images from the Internet and configure them so as to set up a virtual Linux machine that you can subsequently connect to in order to build and test xv6. It'll take a while. Output will look something like this:. When done, try the command " vagrant status " it should produce the following output:. This means the virtual machine is now running. You can connect to it using the command " vagrant ssh ", which will result in the following:. You're now logged into the virtual machine. The following commands will do this:. The first make command should've resulted in a successful build process, while the second should've resulted in the following output:. This last prompt is produced by xv6, which you've now booted into on top of an emulator which is in turn running within a virtual machine. You should now be dropped back into the virtual linux machine. To get out of that and back onto your own machine, just type " exit ". At this point, if you wish, you can terminate the virtual machine by entering " vagrant halt ". To bring it back up just use " vagrant up ". To SSH in to a running virtual machine, use " vagrant ssh ". The following terminal session recording demonstrates the entire procedure described above note that I already downloaded the Linux image earlier, so your " vagrant up " command may take significantly longer to complete. The great thing about using Vagrant is that it automatically gives us the ability to synchronize directory contents between the host machine and the virtual or "guest" machine. This means you can do all your coding with tools installed on your own machine e. The xv6 kernel codebase is already sitting in your cloned repository, so you will simply open and edit any files in there.

Introduction to xv6

This is just a toy example to show you how the plumbing works; system calls are overkill to find the square of a number. The basic idea is to look at how an existing system call is implemented, and replicate that. For instance, uptime is an existing system call in XV6, and I could use grep utility to figure out how that is implemented. From the root of XV6 system, run:. The number increases by 1 as you go downwards. The highlighted line above indicates the relevant change I made. If you look at the source code for syscall. At the end of sysproc. Observe that we use a separate function argptr which is already part of XV6 to extract arguments passed into the function. The parameters of argptr is as follows:. This is where we want to actually declare the function that the user calls. You should already be seeing a bunch of function declarations in there already. Go ahead and declare this function:. This is straightforward. Some things to note here:. This is the final piece of the puzzle. In the file usys. Append our function to this file, as highlighted below:. To see it in action, create a user program any. XV6 System Call 16 Oct Modifications in syscall.

xv6 - System Call Counter

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