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 is it from it's name. When we create `per-CPU` variable, each CPU will have will have it's 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 it's own copy of variable and etc... So every core on multiprocessor will have it's own `GDT` table and every entry in the table will represent a memory segment which can be accessed from the thread which runned on the core. You can read in details about `per-CPU` variables in the [Theory/per-cpu](http://0xax.gitbooks.io/linux-insides/content/Theory/per-cpu.html) post.
Generally per-CPU variables is a 2.6 kernel feature. You can understand what is it from it's name. When we create `per-CPU` variable, each CPU will have will have it's 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 it's own copy of variable and etc... So every core on multiprocessor will have it's own `GDT` table and every entry in the table will represent a memory segment which can be accessed from the thread which runned 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:
Maximilien Richer
9 years ago