[PATCH v2 11/13] 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.

Signed-off-by: Ard Biesheuvel <redacted>
---
 Documentation/arm64/booting.txt  | 12 ++---
 arch/arm64/include/asm/boot.h    |  5 ++
 arch/arm64/include/asm/kvm_mmu.h |  2 +-
 arch/arm64/include/asm/memory.h  | 15 +++---
 arch/arm64/kernel/head.S         |  6 ++-
 arch/arm64/mm/init.c             | 50 +++++++++++++++++++-
 arch/arm64/mm/mmu.c              | 12 +++++
 7 files changed, 86 insertions(+), 16 deletions(-)

diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 701d39d3171a..03e02ebc1b0c 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt

@@ -117,14 +117,14 @@ 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

diff --git a/arch/arm64/include/asm/boot.h b/arch/arm64/include/asm/boot.h
index 81151b67b26b..984cb0fa61ce 100644
--- a/arch/arm64/include/asm/boot.h
+++ b/arch/arm64/include/asm/boot.h

@@ -11,4 +11,9 @@
 #define MIN_FDT_ALIGN		8
 #define MAX_FDT_SIZE		SZ_2M
 
+/*
+ * arm64 requires the kernel image to be 2 MB aligned

+static void __init enforce_memory_limit(void)
+{
+	const phys_addr_t kbase = round_down(__pa(_text), MIN_KIMG_ALIGN);
+	u64 to_remove = memblock_phys_mem_size() - memory_limit;
+	phys_addr_t max_addr = 0;
+	struct memblock_region *r;
+
+	if (memory_limit == (phys_addr_t)ULLONG_MAX)
+		return;
+
+	/*
+	 * The kernel may be high up in physical memory, so try to apply the
+	 * limit below the kernel first, and only let the generic handling
+	 * take over if it turns out we haven't clipped enough memory yet.
+	 */