[PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual memory layout

[Russell King - ARM Linux]

On Mon, Jul 28, 2014 at 10:57:00PM +0400, Konstantin Khlebnikov wrote:
> Also I seen comment somewhere in the code which tells that idrmap pgd is
> always below 4gb which isn't quite true.

"idmap" means "identity map".  It's a region which maps the same value of
physical address to the same value of virtual address.

Since the virtual address space is limited to 4GB, there is /no way/ that
the physical address can be above 4GB, and it still be called an
"identity map".

The reason for this is that code in the identity map will be fetched with
the MMU off.  While this code is running, it will enable the MMU using the
identity map page table pointer, and the CPU must see _no_ change in the
instructions/data fetched from that region.  It will then branch to the
kernel proper, and the kernel will then switch away from the identity page
table.

Once the kernel has switched away from the identity page table, interrupts
and other exceptions can then be taken on the CPU - but not before.
Neither is it expected that the CPU will access any devices until it has
switched away from the identity page table.

What this also means is that the code in the identity map must remain
visible in the low 4GB of physical address space.

> Moreover, I had some experiments with
> mapping ram to random places in qemu and seen that kernel cannot boot if
> PHYS_OFFSET isn't alligned to 128mb which is strange.

That is intentional.  PHYS_OFFSET has a number of restrictions, one of
them is indeed that the physical offset /will/ be aligned to 128MB.
That was decided after looking at the platforms we have and is now
fixed at that value with platform-breaking consequences if it needs
changing.

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