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Update irq_stack initialization

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Replace irq_stack_union with fixed_percpu_data
Update to the current system map
Update description of initialization process
Replace DECLARE macros with the current implementation
This commit is contained in:
Sebastian Fricke 2020-04-05 07:10:37 +02:00
parent 64a9777ca7
commit c96791d527
1 changed files with 15 additions and 7 deletions
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22
Interrupts/linux-interrupts-1.md
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@ -327,13 +327,17 @@ SYM_DATA(initial_gs, .quad INIT_PER_CPU_VAR(fixed_percpu_data))
* `stack_canary` - [Stack canary](http://en.wikipedia.org/wiki/Stack_buffer_overflow#Stack_canaries) for the interrupt stack is a `stack protector`
to verify that the stack hasn't been overwritten. Note that `gs_base` is a 40 bytes array. `GCC` requires that stack canary will be on the fixed offset from the base of the `gs` and its value must be `40` for the `x86_64` and `20` for the `x86`.
The `irq_stack_union` is the first datum in the `percpu` area, we can see it in the `System.map`:
The `fixed_percpu_data` is the first datum in the `percpu` area, we can see it in the `System.map`:
```
0000000000000000 D __per_cpu_start
0000000000000000 D irq_stack_union
0000000000004000 d exception_stacks
0000000000000000 D fixed_percpu_data
00000000000001e0 A kexec_control_code_size
0000000000001000 D cpu_debug_store
0000000000002000 D irq_stack_backing_store
0000000000006000 D cpu_tss_rw
0000000000009000 D gdt_page
000000000000a000 d exception_stacks
...
...
...
@ -342,17 +346,21 @@ The `irq_stack_union` is the first datum in the `percpu` area, we can see it in
We can see its definition in the code:
```C
DECLARE_PER_CPU_FIRST(union irq_stack_union, irq_stack_union) __visible;
DECLARE_PER_CPU_FIRST(struct fixed_percpu_data, fixed_percpu_data) __visible;
```
Now, it's time to look at the initialization of the `irq_stack_union`. Besides the `irq_stack_union` definition, we can see the definition of the following per-cpu variables in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/processor.h):
Now, it's time to look at the initialization of the `fixed_percpu_data`. Besides the `fixed_percpu_data` definition, we can see the definition of the following per-cpu variables in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/processor.h):
```C
DECLARE_PER_CPU(char *, irq_stack_ptr);
DECLARE_PER_CPU(struct irq_stack *, hardirq_stack_ptr);
...
DECLARE_PER_CPU(unsigned int, irq_count);
...
/* Per CPU softirq stack pointer */
DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
```
The first is the `irq_stack_ptr`. From the variable's name, it is obvious that this is a pointer to the top of the stack. The second - `irq_count` is used to check if a CPU is already on an interrupt stack or not. Initialization of the `irq_stack_ptr` is located in the `setup_per_cpu_areas` function in [arch/x86/kernel/setup_percpu.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/setup_percpu.c):
The first and third are the stack pointers for hardware and software interrupts. It is obvious from the name of the variables, that these point to the top of stacks. The second - `irq_count` is used to check if a CPU is already on an interrupt stack or not. Initialization of the `irq_stack_ptr` is located in the `setup_per_cpu_areas` function in [arch/x86/kernel/setup_percpu.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/setup_percpu.c):
```C
void __init setup_per_cpu_areas(void)