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, 5 insertions, 1 deletions

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 04d38a058b19..05b98289093e 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -428,7 +428,11 @@ __mmap_switched:
 	and	x4, x4, #~(THREAD_SIZE - 1)
 	msr	sp_el0, x4			// Save thread_info
 	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
-	str_l	x24, memstart_addr, x6		// Save PHYS_OFFSET
+
+	ldr	x4, =KIMAGE_VADDR		// Save the offset between
+	sub	x4, x4, x24			// the kernel virtual and
+	str_l	x4, kimage_voffset, x5		// physical mappings
+
 	mov	x29, #0
 #ifdef CONFIG_KASAN
 	bl	kasan_early_init