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For historical reasons, the arm64 kernel VA space is configured as two
equally sized halves, i.e., on a 48-bit VA build, the VA space is split
into a 47-bit vmalloc region and a 47-bit linear region.
When support for 52-bit virtual addressing was added, this equal split
was kept, resulting in a substantial waste of virtual address space in
the linear region:
48-bit VA 52-bit VA
0xffff_ffff_ffff_ffff +-------------+ +-------------+
| vmalloc | | vmalloc |
0xffff_8000_0000_0000 +-------------+ _PAGE_END(48) +-------------+
| linear | : :
0xffff_0000_0000_0000 +-------------+ : :
: : : :
: : : :
: : : :
: : : currently :
: unusable : : :
: : : unused :
: by : : :
: : : :
: hardware : : :
: : : :
0xfff8_0000_0000_0000 : : _PAGE_END(52) +-------------+
: : | |
: : | |
: : | |
: : | |
: : | |
: unusable : | |
: : | linear |
: by : | |
: : | region |
: hardware : | |
: : | |
: : | |
: : | |
: : | |
: : | |
: : | |
0xfff0_0000_0000_0000 +-------------+ PAGE_OFFSET +-------------+
As illustrated above, the 52-bit VA kernel uses 47 bits for the vmalloc
space (as before), to ensure that a single 64k granule kernel image can
support any 64k granule capable system, regardless of whether it supports
the 52-bit virtual addressing extension. However, due to the fact that
the VA space is still split in equal halves, the linear region is only
2^51 bytes in size, wasting almost half of the 52-bit VA space.
Let's fix this, by abandoning the equal split, and simply assigning all
VA space outside of the vmalloc region to the linear region.
The KASAN shadow region is reconfigured so that it ends at the start of
the vmalloc region, and grows downwards. That way, the arrangement of
the vmalloc space (which contains kernel mappings, modules, BPF region,
the vmemmap array etc) is identical between non-KASAN and KASAN builds,
which aids debugging.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Link: https://lore.kernel.org/r/20201008153602.9467-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.