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authorArd Biesheuvel <ard.biesheuvel@linaro.org>2016-02-16 13:52:42 +0100
committerCatalin Marinas <catalin.marinas@arm.com>2016-02-18 18:16:53 +0000
commita7f8de168ace487fa7b88cb154e413cf40e87fc6 (patch)
tree870a068dba048cdf795c241e97f1e8684fed2208 /Documentation/arm64
parenta89dea585371a9d5d85499db47c93f129be8e0c4 (diff)
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arm64: allow kernel Image to be loaded anywhere in physical memory
This relaxes the kernel Image placement requirements, so that it may be placed at any 2 MB aligned offset in physical memory. This is accomplished by ignoring PHYS_OFFSET when installing memblocks, and accounting for the apparent virtual offset of the kernel Image. As a result, virtual address references below PAGE_OFFSET are correctly mapped onto physical references into the kernel Image regardless of where it sits in memory. Special care needs to be taken for dealing with memory limits passed via mem=, since the generic implementation clips memory top down, which may clip the kernel image itself if it is loaded high up in memory. To deal with this case, we simply add back the memory covering the kernel image, which may result in more memory to be retained than was passed as a mem= parameter. Since mem= should not be considered a production feature, a panic notifier handler is installed that dumps the memory limit at panic time if one was set. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'Documentation/arm64')
-rw-r--r--Documentation/arm64/booting.txt20
1 files changed, 13 insertions, 7 deletions
diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 701d39d3171a..56d6d8b796db 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -109,7 +109,13 @@ Header notes:
1 - 4K
2 - 16K
3 - 64K
- Bits 3-63: Reserved.
+ Bit 3: Kernel physical placement
+ 0 - 2MB aligned base should be as close as possible
+ to the base of DRAM, since memory below it is not
+ accessible via the linear mapping
+ 1 - 2MB aligned base may be anywhere in physical
+ memory
+ Bits 4-63: Reserved.
- When image_size is zero, a bootloader should attempt to keep as much
memory as possible free for use by the kernel immediately after the
@@ -117,14 +123,14 @@ Header notes:
depending on selected features, and is effectively unbound.
The Image must be placed text_offset bytes from a 2MB aligned base
-address near the start of usable system RAM and called there. Memory
-below that base address is currently unusable by Linux, and therefore it
-is strongly recommended that this location is the start of system RAM.
-The region between the 2 MB aligned base address and the start of the
-image has no special significance to the kernel, and may be used for
-other purposes.
+address anywhere in usable system RAM and called there. The region
+between the 2 MB aligned base address and the start of the image has no
+special significance to the kernel, and may be used for other purposes.
At least image_size bytes from the start of the image must be free for
use by the kernel.
+NOTE: versions prior to v4.6 cannot make use of memory below the
+physical offset of the Image so it is recommended that the Image be
+placed as close as possible to the start of system RAM.
Any memory described to the kernel (even that below the start of the
image) which is not marked as reserved from the kernel (e.g., with a