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>

1 files changed, 13 insertions, 7 deletions

diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 701d39d..56d6d8b 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