[PATCH v5sub1 8/8] arm64: allow kernel Image to be loaded anywhere in physical memory

On Mon, Feb 01, 2016 at 05:31:11PM +0100, Ard Biesheuvel wrote:

On 1 February 2016 at 16:13, Ard Biesheuvel [off-list ref] wrote:

quoted

On 1 February 2016 at 16:06, Catalin Marinas [off-list ref] wrote:

quoted

On Mon, Feb 01, 2016 at 11:54:53AM +0100, Ard Biesheuvel wrote:

quoted

Note that limiting memory using mem= is not unambiguous anymore after
this change, considering that the kernel may be at the top of physical
memory, and clipping from the bottom rather than the top will discard
any 32-bit DMA addressable memory first. To deal with this, the handling
of mem= is reimplemented to clip top down, but take special care not to
clip memory that covers the kernel image.

I may have forgotten the reason - why do we need to avoid clipping the
memory that covers the kernel image? It's already mapped in the vmalloc
area, so we wouldn't need it in the linear map as well.

Good question. Originally, I needed it for swapper_pg_dir, whose
pud/pmd/pte levels were accessed via __va() translations of the values
found in the higher-up table entries, but after Mark's patches, only
the top level pgd of swapper_pg_dir is still used. Similarly, for
idmap_pg_dir, we don't change any mappings at runtime so the same
applies there I think.

I will try dropping this, and see what happens.

I have given this a spin, and this chokes on
a) the fact that not all of the translation tables are accessible via
the linear mapping: the fixmap, due to its vicinity to PCI i/o and
other populated regions, will share its pud/pmd level tables with
other users, like ioremap, which traverses the translation tables in
the ordinary way, i.e., it expects that __va() applied on the phys
address in the table entry returns something that is mapped

Ah, __va(__pa(x)) is not an identity function and I don't think it's
worth fixing it (the __pa() case is much simpler). But it also means
that we won't be able to remove the kernel image alias in the linear
mapping. It shouldn't be a problem for KASLR as long as we randomise
both kernel image PA and VA.

b) free_initmem() now calls __free_pages() on a region that we never
mapped or registered as available.

So it may be feasible with some hackery, but I wonder if it is worth
it to complicate the common case for implementing mem= more
efficiently.

I don't care about efficiency, I was hoping to avoid the additional
arm64-specific memory clipping but it seems that it could easily get
more complicated. So let's leave as it is.

Consider this sub-series merged (I'll push it to -next around -rc3).

Thanks.

-- 
Catalin