arm64: kexec: pass kimage as the only argument to relocation function

Currently, kexec relocation function (arm64_relocate_new_kernel) accepts
the following arguments:

head:		start of array that contains relocation information.
entry:		entry point for new kernel or purgatory.
dtb_mem:	first and only argument to entry.

The number of arguments cannot be easily expended, because this
function is also called from HVC_SOFT_RESTART, which preserves only
three arguments. And, also arm64_relocate_new_kernel is written in
assembly but called without stack, thus no place to move extra arguments
to free registers.

Soon, we will need to pass more arguments: once we enable MMU we
will need to pass information about page tables.

Pass kimage to arm64_relocate_new_kernel, and teach it to get the
required fields from kimage.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210930143113.1502553-8-pasha.tatashin@soleen.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/relocate_kernel.S

@@ -27,9 +27,7 @@
  */
 SYM_CODE_START(arm64_relocate_new_kernel)
 	/* Setup the list loop variables. */
-	mov	x18, x2				/* x18 = dtb address */
-	mov	x17, x1				/* x17 = kimage_start */
-	mov	x16, x0				/* x16 = kimage_head */
+	ldr	x16, [x0, #KIMAGE_HEAD]		/* x16 = kimage_head */
 	mov	x14, xzr			/* x14 = entry ptr */
 	mov	x13, xzr			/* x13 = copy dest */
 	raw_dcache_line_size x15, x1		/* x15 = dcache line size */
@@ -63,12 +61,12 @@ SYM_CODE_START(arm64_relocate_new_kernel)
 	isb
 
 	/* Start new image. */
-	mov	x0, x18
+	ldr	x4, [x0, #KIMAGE_START]		/* relocation start */
+	ldr	x0, [x0, #KIMAGE_ARCH_DTB_MEM]	/* dtb address */
 	mov	x1, xzr
 	mov	x2, xzr
 	mov	x3, xzr
-	br	x17
-
+	br	x4
 SYM_CODE_END(arm64_relocate_new_kernel)
 
 .align 3	/* To keep the 64-bit values below naturally aligned. */