[PATCH v3] arm64: mm: move zero page from .bss to right before swapper_pg_dir

[mark.rutland@arm.com (Mark Rutland)]

Hi,

On Mon, Sep 12, 2016 at 05:15:25PM +0100, Ard Biesheuvel wrote:
> Move the statically allocated zero page from the .bss section to right
> before swapper_pg_dir. This allows us to refer to its physical address
> by simply reading TTBR1_EL1 (which always points to swapper_pg_dir and
> always has its ASID field cleared), and subtracting PAGE_SIZE.

It might be worth worth mentioning that we want to do this to make
cpu_set_reserved_ttbr0() as cheap as possible for the TTBR0_SW_PAN
stuff, as that'll mean we're calling it far more frequently.

> To protect the zero page from inadvertent modification, carve out a
> segment that covers it as well as idmap_pg_dir[], and mark it read-only
> in both the primary and the linear mappings of the kernel.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Otherwise, this looks good to me, builds without warnings, and works on
Juno and Seattle without issue even when I throw the usual set of
problematic config options at it. Which is to say:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.