How are the fs/gs registers used in Linux AMD64?

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On the x86-64 architecture, two registers have a special purpose: FS and GS. In linux 2.6.*, the FS register seem to be used to store thread-local information.

  • Is that correct?
  • What is stored at fs:0? Is there any C structure that describe this content?
  • What is then the use of GS?
x86-64 x86 assembly architecture linux
3 Answers

What is then the use of GS?

x86_64 Linux kernel uses GS register as a efficiency way to acquire kernel space stack for system calls.

GS register stores the base address for per-cpu area. To acquire the kernel space stack, in entry_SYSCALL_64

movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

After expanding PER_CPU_VAR, we get the following:

movq    %gs:cpu_current_top_of_stack, %rsp
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