What is kernel space in linux

Linux kernel space and user space

Thanks in advance for any input, literature/links that can help clarify my questions are okay as well.

2 Answers 2

You’ve got the general idea mostly right, but make this adjustment: there’s only one «kernelspace» for the whole machine, and all processes share it.

When a process is active, it can either be running in «user mode» or «kernel mode».

In user mode, the instructions being executed by the CPU are in the userspace side of the memory map. The program is running its own code, or code from a userspace library. In user mode, a process has limited abilities. There is a flag in the CPU which tells it not to allow the use of privileged instructions, and kernel memory, although it exists in the process’s memory map, is inaccessible. (You wouldn’t want let any program just read and write the kernel’s memory — all security would be gone.)

When a process wants to do something other than move data around in its own (userspace) virtual memory, like open a file for example, it must make a syscall. Each CPU architecture has its own unique quirky method of making syscalls, but they all boil down to this: a magic instruction is executed, the CPU turns on the «privileged mode» flag, and jumps to a special address in kernelspace, the «syscall entry point».

Now the process is running in kernel mode. Instructions being executed are located in kernel memory, and they can read and write any memory they want to. The kernel examines the request that the process just made and decides what to do with it.

In the open example, the kernel receives 2 or 3 parameters corresponding to the arguments of int open(const char *filename, int flags[, int mode]) . The first argument provides an example of when kernelspace needs access to userspace. You said open(«foo», O_RDONLY) so the string «foo» is part of your program in userspace. The syscall mechanism only passed a pointer, not a string, so the kernel must read the string from user memory.

To find the requested file, the kernel may consult with filesystem drivers (to figure out where the file is) and block device drivers (to load the necessary blocks from disk) or network device drivers and protocols (to load the file from a remote source). All of those things are part of the kernel, i.e. in kernelspace, regardless of whether they are built-in or were loaded as modules.

If the request can’t be satisfied immediately, the kernel may put the process to sleep. That means the process will be taken off the CPU until a response is received from the disk or network. Another process may get a chance to run now. Later, when the response comes in, your process starts running again (still in kernel mode). Now that it’s found the file, the open syscall can finish up (check the permissions, create a file descriptor) and return to userspace.

Returning to userspace is a simple matter of putting the CPU back in non-privileged mode and restoring the registers to what they were before the user->kernel transition, with the instruction pointer pointing at the instruction after the magic syscall instruction.

Besides syscalls, there are other things that can cause a transition from user mode to kernel mode, including:

1. page faults — if your process accesses a virtual memory address that doesn’t have a physical address assigned to it, the CPU enters kernel mode and jumps to the page fault handler. The kernel then decides whether the virtual address is valid or not, and it either creates a physical page and resumes the process in userspace where it left off, or sends a SIGSEGV.
2. interrupts — some hardware (network, disk, serial port, etc.) notifies the CPU that it requires attention. The CPU enters kernel mode and jumps to a handler, the kernel responds to it and then resumes the userspace process that was running before the interrupt.

Loading a module is done with a syscall that asks the kernel to copy the module’s code and data into kernelspace and run its initialization code in kernel mode.

This is pretty long, so I’m stopping. I hope the walk-through focusing on user-kernel transitions has provided enough examples to solidify the idea.

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What is difference between User space and Kernel space?

Is Kernel space used when Kernel is executing on the behalf of the user program i.e. System Call? Or is it the address space for all the Kernel threads (for example scheduler)? If it is the first one, than does it mean that normal user program cannot have more than 3GB of memory (if the division is 3GB + 1GB)? Also, in that case how can kernel use High Memory, because to what virtual memory address will the pages from high memory be mapped to, as 1GB of kernel space will be logically mapped?

3 Answers 3

Is Kernel space used when Kernel is executing on the behalf of the user program i.e. System Call? Or is it the address space for all the Kernel threads (for example scheduler)?

Before we go any further, we should state this about memory.

Memory gets divided into two distinct areas:

• The user space, which is a set of locations where normal user processes run (i.e everything other than the kernel). The role of the kernel is to manage applications running in this space from messing with each other, and the machine.
• The kernel space, which is the location where the code and data of the kernel is stored, and executes under.

Processes running under the user space have access only to a limited part of memory, whereas the kernel has access to all of the memory. Processes running in user space also don’t have access to the kernel space. User space processes can only access a small part of the kernel via an interface exposed by the kernel — the system calls. If a process performs a system call, a software interrupt is sent to the kernel, which then dispatches the appropriate interrupt handler and continues its work after the handler has finished.

Kernel space code has the property to run in «kernel mode», which (in your typical desktop -x86- computer) is what you call code that executes under ring 0. Typically in x86 architecture, there are 4 rings of protection. Ring 0 (kernel mode), Ring 1 (may be used by virtual machine hypervisors or drivers), Ring 2 (may be used by drivers, I am not so sure about that though). Ring 3 is what typical applications run under. It is the least privileged ring, and applications running on it have access to a subset of the processor’s instructions. Ring 0 (kernel space) is the most privileged ring, and has access to all of the machine’s instructions. For an example of this, a «plain» application (like a browser) can not use x86 assembly instructions lgdt to load the global descriptor table, nor hlt to halt a processor.

If it is the first one, than does it mean that normal user program cannot have more than 3GB of memory (if the division is 3GB + 1GB)? Also, in that case how can kernel use High Memory, because to what virtual memory address will the pages from high memory be mapped to, as 1GB of kernel space will be logically mapped?

For an answer to this, please refer to the excellent answer by wag to What are high memory and low memory on Linux?.

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What is the Difference Between User Space and Kernel Space in Linux

In a Linux operating system, the kernel is responsible for managing system resources such as memory, processors, and input/output operations. The kernel space is the privileged area of the system where the kernel resides and manages system resources. On the other hand, the user space is the area where user applications and processes run.

This article will explain the differences between user and kernel space in detail by discussing the following content.

• What is User Space?
• What is Kernel Space?
• Key Differences between User Space and Kernel Space
• Understanding System Calls
• Importance of User Space and Kernel Space in System Performance

What is User Space?

The user space is part of the system where user applications and processes run. It is a non-privileged area of the system, meaning user processes do not directly access system resources. Instead, they communicate with the kernel through system calls. User processes are executed in a protected memory area, which isolates them from other processes running in the system.

What is Kernel Space?

The kernel space is the privileged area of the system where the kernel resides. It directly accesses system resources such as memory, I/O operations, and hardware devices. The kernel handles the management and provision of system resources to user processes through system calls. In contrast, the kernel space is accountable for ensuring system security and stability. These are the fundamental distinctions between user space and kernel space.

Key Differences Between User Space and Kernel Space

Some of the major key differences between user and kernel space are:

• Access to System Resources: The key difference between user space and kernel space is the level of access they have to system resources. User processes do not have direct access to system resources and must use system calls to interact with the kernel. On the other hand, the kernel has direct access to system resources and can manage them directly.
• Privileges: Another difference between user space and kernel space is the level of privileges they have. User processes run in a non-privileged mode, which means that they do not have the same level of control over the system as the kernel. The kernel runs in a privileged mode, which means that it has full control over system resources.
• Memory Management: Memory management is another area where user space and kernel space differ. User processes have limited memory access and can only access the memory that has been allocated to them. On the other hand, the kernel has full memory access and can allocate and deallocate memory as needed.
• Execution Time: Execution time is another important factor to consider when comparing user space and kernel space. User processes have a limited amount of execution time. If they exceed that time, they may be terminated by the system. The kernel, on the other hand, has no such restrictions and can execute for an indefinite amount of time.
• System Stability: The overall stability of the system is also affected by the differences in user space and kernel space. User processes run in a protected environment, which isolates them from other processes running in the system. This helps to ensure the stability of the system, as a malfunctioning user process will not affect other processes in the system. The kernel, on the other hand, has direct access to system resources and is responsible for managing them. Any malfunction in the kernel can have a significant impact on the overall stability of the system.

Understanding System Calls

System calls are the primary means of communication between user processes and the kernel in a Linux operating system. User processes use system calls to request services from the kernel, such as allocating memory, accessing I/O devices, and creating new processes. The kernel, in turn, provides these services to the user processes through system calls.

System calls provide a level of abstraction between user processes and the kernel. This allows user processes to interact with the system in a standardized way without needing to know the low-level details of how the system works.

Conclusion

The user and kernel spaces are two different spaces in the Linux operating system. The user space is used to run user applications, while the kernel space is reserved for the core functionality of the operating system. The kernel space has full access to the system hardware and is responsible for managing system resources and providing services. Additional differences between these two have been discussed in detail in this article.

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