Linux kernel assembly and logic

10
votes

My question is somewhat weird but I will do my best to explain.

Looking at the languages the linux kernel has, I got C and assembly even though I read a text that said [quote] Second iteration of Unix is written completely in C [/quote]

I thought that was misleading but when I said that kernel has assembly code I got 2 questions of the start

  1. What assembly files are in the kernel and what's their use?
  2. Assembly is architecture dependant so how can linux be installed on more than one CPU architecture

And if linux kernel is truly written completely in C than how can it get GCC needed for compiling?

I did a complete find / -name *.s and just got one assembly file (asm-offset.s) somewhere in the /usr/src/linux-headers-`uname -r/

Somehow I don't think that is helping with the GCC working, so how can linux work without assembly or if it uses assembly where is it and how can it be stable when it depends on the arch.

Thanks in advance

5
linuxassemblykernelinline-assembly
Architecture-dependent files are in the arch directory, for x86 they have the .S (uppercase) extension. For example, the following link gives you the assembly source to implement VDSO on x86 lxr.free-electrons.com/source/arch/x86/vdso/vdso.S - amdn
Do you even have the linux source installed? The headers won't have much code in them. Also search for *.S (capital) as those are assembly files too. The C code also has some inline assembly and other things that are not standard C, thus the gcc requirement. - Jester
@Jester You are right about the sources. I dont have them installed. But still the question remains. What assembly files are needed by the kernel (i know about head.S) But how is it arch independant ? - daniels_pa

5 Answers

12
votes

1. Why assembly is used?

Because there are certain things then can be done only in assembly and because assembly results in a faster code. For eg, "you can get access to unusual programming modes of your processor (e.g. 16 bit mode to interface startup, firmware, or legacy code on Intel PCs)". Read here for more reasons.

2. What assembly file are used?

From: https://www.kernel.org/doc/Documentation/arm/README

"The initial entry into the kernel is via head.S, which uses machine independent code. The machine is selected by the value of 'r1' on entry, which must be kept unique."

From https://www.ibm.com/developerworks/library/l-linuxboot/

"When the bzImage (for an i386 image) is invoked, you begin at ./arch/i386/boot/head.S in the start assembly routine (see Figure 3 for the major flow). This routine does some basic hardware setup and invokes the startup_32 routine in ./arch/i386/boot/compressed/head.S. This routine sets up a basic environment (stack, etc.) and clears the Block Started by Symbol (BSS). The kernel is then decompressed through a call to a C function called decompress_kernel (located in ./arch/i386/boot/compressed/misc.c). When the kernel is decompressed into memory, it is called. This is yet another startup_32 function, but this function is in ./arch/i386/kernel/head.S."

Apart from these assembly files, lot of linux kernel code has usage of inline assembly.

3. Architecture dependence?

And you are right about it being architecture dependent, that's why the linux kernel code is ported to different architecture.

8
votes

Things written mainly in assembly in Linux:

  • Boot code: boots up the machine and sets it up in a state in which it can start executing C code (e.g: on some processors you may need to manually initialize caches and TLBs, on x86 you have to switch to protected mode, ...)
  • Interrupts/Exceptions/Traps entry points/returns: there you need to do very processor-specific things, e.g: saving registers and reenabling interrupts, and eventually restoring registers and properly returning to user mode. Some exceptions may be handled entirely in assembly.
  • Instruction emulation: some CPU models may not support certain instructions, may not support unaligned data access, or may not have an FPU. An option is using emulation when getting the corresponding exception.
  • VDSO: the VDSO is a virtual library that the kernel maps into userspace. It allows e.g: selecting the optimal syscall sequence for the current CPU (on x86 use sysenter/syscall instead of int 0x80 if available), and implementing certain system calls without requiring a context switch (e.g: gettimeofday()).
  • Atomic operations and locks: Maybe in a future some of these could be written using C11 support for atomic operations.
  • Copying memory from/to user mode: Besides using an optimized copy, these check for out-of-bounds access.
  • Optimized routines: the kernel has optimized version of some routines, e.g: crypto routines, memset, clear_page, csum_copy (checksum and copy to another place IP data in one pass), ...
  • Support for suspend/resume and other ACPI/EFI/firmware thingies
  • BPF JIT: newer kernels include a JIT compiler for BPF expressions (used for example by tcpdump, secmode mode 2, ...)
  • ...

To support different architectures, Linux has assembly code (re-)written for each architecture it supports (and sometimes, there are several implementations of some code for different platforms using the same CPU architecture). Just look at all the subdirectories under arch/

4
votes

Assembly is needed for a couple of reasons.

  1. There are many instructions that are needed for the operation of an operating system that have no C equivalent, at least on most processors. A good example on Intel x86/64 processors is the iret instruciton, which returns from hardware/software interrupts. These interrupts are key to handling hardware events (like a keyboard press) and system calls from programs on older processors.
  2. A computer does not start up in a state that is immediately ready for execution of C code. For an Intel example, when execution gets to the startup routine the processor may not be in 32-bit mode (or 64-bit mode), and the stack required by C also may not be ready. There are some other features present in some processors (like paging) which need to be turned on from assembly as well.

However, most of the Linux kernel is written in C, which interfaces with some platform specific C/assembly code through standardized interfaces. By separating the parts in this way, most of the logic of the Linux kernel can be shared between platforms. The build system simply compiles the platform independent and dependent parts together for specific platforms, which results in different executable kernel files for different platforms (and kernel configurations for that matter).

0
votes

Assembly code in the kernel is generally used for low-level hardware interaction that can't be done directly from C. They're like a platform- specific foundation that's used by higher-level parts of the kernel that are written in C.

The kernel source tree contains assembly code for a variety of systems. When you compile a kernel for a particular type of system (such as an x86 PC), only the appropriate assembly code for that platform is included in the build process.

0
votes

Linux is not the second version of Unix (or Unix in general). It is Unix compatible, but Unix and Linux have separate histories and, in terms of code base (of their kernels), are completely separate. Linus Torvald's idea was to write an open source Unix.

Some of the lower level things like some of the architecture dependent parts of memory management are done in assembly. The old (but still available) Linux kernel API for x86, int 0x80, is implemented in assembly. There are probably other places in the kernel that are implemented in assembly, but I don't know any others.

When you compile the kernel, you select an architecture to target. Depending on the target, the right assembly files for that architecture are included in the build.

The reason you don't find anything is because you're searching the headers, not the sources. Download a tar ball from kernel.org and search that.