On Mon, Mar 31, 2014 at 12:31:14PM +0100, Catalin Marinas wrote:

On Mon, Mar 31, 2014 at 07:56:53AM +0100, Arnd Bergmann wrote:

quoted

On Monday 31 March 2014 12:51:07 Jungseok Lee wrote:

quoted

Current ARM64 kernel cannot support 4KB pages for 40-bit physical address
space described in [1] due to one major issue + one minor issue.

Firstly, kernel logical memory map (0xffffffc000000000-0xffffffffffffffff)
cannot cover DRAM region from 544GB to 1024GB in [1]. Specifically, ARM64
kernel fails to create mapping for this region in map_mem function
(arch/arm64/mm/mmu.c) since __phys_to_virt for this region reaches to
address overflow. I've used 3.14-rc8+Fast Models to validate the statement.

[...]

quoted

a) always use a four-level page table in kernel space, regardless of
whether we do it in user space. We can move the kernel mappings down
in address space either by one 512GB entry to 0xffffff0000000000, or
to match the 64k-page location at 0xfffffc0000000000, or all the way
to to 0xfffc000000000000. In any case, we can have all the dynamic
mappings within one 512GB area and pretend we have a three-level
page table for them, while the rest of DRAM is mapped statically at
early boot time using 512GB large pages.

That's a workaround but we end up with two (or more) kernel pgds - one
for vmalloc, ioremap etc. and another (static) one for the kernel linear
mapping. So far there isn't any memory mapping carved out but we have to
be careful in the future.

However, kernel page table walking would be a bit slower with 4-levels.

Yet another approach would be to enable 4-levels of page tables (no
nopud.h) in the kernel with pgd_offset_k doing the right thing for 4
levels but user space configured to 3-levels only and pgd_offset
returning 0 while pud_offset does what pgd_offset currently implements
for 3 levels.

This solves the page table walk latency for user but not for kernel.
Anyway, if the hardware memory map is so sparse (a real SoC, not the
spec), I don't think we have other ways to support it with 3-levels of
page table for the kernel, unless we hack __virt_to_phys/__phys_to_virt.

-- 
Catalin