Here we go over all the CPUs on-by-one and setup `irq_stack_ptr`. This turns out to be equal to the top of the interrupt stack minus `64`. Why `64`? If you remember, we set the stack canary in the beginning of the `start_kernel` function from the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) with the call of the `boot_init_stack_canary` function:
Note that `canary` is `64` bits value. That's why we need to subtract `64` from the size of the interrupt stack to avoid overlapping with the stack canary value. Initialization of the `irq_stack_union.gs_base` is in the `load_percpu_segment` function from the [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c):
TODO maybe more about the wrmsl
Here we go over all the CPUs on-by-one and setup `irq_stack_ptr`. This turns out to be equal to the top of the interrupt stack minus `64`. Why `64`?TODO [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c) source code file is following: