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docs: documented process isolation concepts

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antifallobst 2023-03-07 20:11:25 +01:00
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@ -115,6 +115,37 @@ No detailed Information available (cpu_state null reference)
```
The `Error Message` could still be helpful, but good luck finding that bug.
## Memory Layout
NoxOS uses a higher half kernel, this means, that the kernels executable is mapped above `0x800000000000`.
All kernel and bootloader resources are also mapped in the higher half.
This leaves the lower half for process specific mappings
## Process Memory Isolation
Every process has its own virtual memory space (page map).
This space contains always the following mappings (except in the kernels main process):
| Name | Address | Permissions |
|---------------------------|--------------------|-------------------|
| Process executable | 0x0000010000000000 | as defined in ELF |
| Thread data (stack, etc.) | 0x0000010100000000 | Read/Write |
| Kernel executable | 0xFFFFFFFF80000000 | Read/Exec |
| Kernel Heap | 0xFFFFFFFFF0000000 | Read/Write |
### Thread data
Every thread has a _Thread Data_ region, that contains thread specific stuff like a stack.
The first threads _Thread Data_ region is at `0x0000010100000000`, the seconds at `0x0000010200000000` and so on.
In this setup, every _Thread Data_ region has a virtual size of 4GB.
### Context switching
Switching between threads is a bit tricky, when the threads stacks are in different page maps.
When performing a context switch, the memory region `0x0000010000000000` <--> `0x0000020000000000`
is dirty mapped (the PML4 entries are copied) from the next processes' into the kernels page map.
This region contains all _Thread Data_ regions and the processes' executable mappings.
Due to this dirty mapping the next interrupt handler can always access the current processes' or threads' data.
## Syscalls
NoxOS will use interrupt based syscalls.
To perform a syscall, write its ID into the `rax` register and call interrupt 0x80.