On Fri, Apr 22, 2016 at 09:16:12AM +0200, Ingo Molnar wrote:
>
> * Kees Cook <keesc...@chromium.org> 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.
So it wouldn't hurt to have a more detailed explanation like this one in
the text. And the 2G thing confused me maybe because it actually means
32-bit: 0x8000_0000 is 2G and is negative since the MSB is 1b. And I was
wondering: "but what about 64-bit...?"
Thanks.
--
Regards/Gruss,
Boris.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284
(AG Nürnberg)
--
