arm64: vmlinux.ld.S: add assertion for tramp_pg_dir offset

Add TRAMP_SWAPPER_OFFSET and use that instead of hardcoding
the offset between swapper_pg_dir and tramp_pg_dir.

Then use TRAMP_SWAPPER_OFFSET to assert that the offset is
correct at link time.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210202123658.22308-3-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/memory.h

@@ -165,6 +165,12 @@
  */
 #define RESERVED_SWAPPER_OFFSET	(PAGE_SIZE)
 
+/*
+ *  Open-coded (swapper_pg_dir - tramp_pg_dir) as this cannot be calculated
+ *  until link time.
+ */
+#define TRAMP_SWAPPER_OFFSET	(2 * PAGE_SIZE)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/bitops.h>

arch/arm64/kernel/entry.S

@@ -805,7 +805,7 @@ SYM_CODE_END(ret_to_user)
 	// Move from tramp_pg_dir to swapper_pg_dir
 	.macro tramp_map_kernel, tmp
 	mrs	\tmp, ttbr1_el1
-	add	\tmp, \tmp, #(2 * PAGE_SIZE)
+	add	\tmp, \tmp, #TRAMP_SWAPPER_OFFSET
 	bic	\tmp, \tmp, #USER_ASID_FLAG
 	msr	ttbr1_el1, \tmp
 #ifdef CONFIG_QCOM_FALKOR_ERRATUM_1003
@@ -825,7 +825,7 @@ alternative_else_nop_endif
 	// Move from swapper_pg_dir to tramp_pg_dir
 	.macro tramp_unmap_kernel, tmp
 	mrs	\tmp, ttbr1_el1
-	sub	\tmp, \tmp, #(2 * PAGE_SIZE)
+	sub	\tmp, \tmp, #TRAMP_SWAPPER_OFFSET
 	orr	\tmp, \tmp, #USER_ASID_FLAG
 	msr	ttbr1_el1, \tmp
 	/*

arch/arm64/kernel/vmlinux.lds.S

@@ -319,3 +319,8 @@ ASSERT(_text == KIMAGE_VADDR, "HEAD is misaligned")
 
 ASSERT(swapper_pg_dir - reserved_pg_dir == RESERVED_SWAPPER_OFFSET,
        "RESERVED_SWAPPER_OFFSET is wrong!")
+
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+ASSERT(swapper_pg_dir - tramp_pg_dir == TRAMP_SWAPPER_OFFSET,
+       "TRAMP_SWAPPER_OFFSET is wrong!")
+#endif