As we got `init_per_cpu__gdt_page` in `INIT_PER_CPU_VAR` and `INIT_PER_CPU` macro from linker script will be expanded we will get offset from the `__per_cpu_load`. After this calculations, we will have correct base address of the new GDT.
Generally per-CPU variables is a 2.6 kernel feature. You can understand what it is from its name. When we create `per-CPU` variable, each CPU will have will have its own copy of this variable. Here we creating `gdt_page` per-CPU variable. There are many advantages for variables of this type, like there are no locks, because each CPU works with its own copy of variable and etc... So every core on multiprocessor will have its own `GDT` table and every entry in the table will represent a memory segment which can be accessed from the thread which ran on the core. You can read in details about `per-CPU` variables in the [Theory/per-cpu](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html) post.
Generally per-CPU variables is a 2.6 kernel feature. You can understand what it is from its name. When we create `per-CPU` variable, each CPU will have its own copy of this variable. Here we creating `gdt_page` per-CPU variable. There are many advantages for variables of this type, like there are no locks, because each CPU works with its own copy of variable and etc... So every core on multiprocessor will have its own `GDT` table and every entry in the table will represent a memory segment which can be accessed from the thread which ran on the core. You can read in details about `per-CPU` variables in the [Theory/per-cpu](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html) post.
As we loaded new Global Descriptor Table, we reload segments as we did it every time:
Radek Dostál
7 years ago