* Kees Cook <[email protected]> wrote:
> >> + Since the kernel is built using 2GB addressing,
> >
> > Does that try to refer to the 1G kernel and 1G fixmap pagetable
> > mappings? I.e., level2_kernel_pgt and level2_fixmap_pgt in
> > arch/x86/kernel/head_64.S?
>
> The "2GB addressing" part is in reference to:
>
> -mcmodel=kernel
> Generate code for the kernel code model. The kernel runs in the
> negative 2 GB of the address space. This model has to be used for
> Linux kernel code.
On x86-64 this is a special GCC compiler small memory model, it is called the
'kernel code model', which is rather generic and no 'real name' ever stuck.
Due to RIP-relative addressing and the sign-extension of 48 bit virtual
addresses,
this allows nearly as compact kernel code and (static) kernel data definitions
as
a 32-bit kernel would allow.
The (positive) 0-4GB virtual memory range has similar advantages, but is of
course
frequently used by user-space code. Negative addresses are reserved for the
kernel
only.
Thanks,
Ingo
