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

Add RESERVED_SWAPPER_OFFSET and use that instead of hardcoding
the offset between swapper_pg_dir and reserved_pg_dir.

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

Signed-off-by: Joey Gouly <[e-mail]>
Reviewed-by: Mark Rutland <[e-mail]>
Tested-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/asm-uaccess.h

···
 	.macro	__uaccess_ttbr0_disable, tmp1
 	mrs	\tmp1, ttbr1_el1			// swapper_pg_dir
 	bic	\tmp1, \tmp1, #TTBR_ASID_MASK
-	sub	\tmp1, \tmp1, #PAGE_SIZE		// reserved_pg_dir just before swapper_pg_dir
+	sub	\tmp1, \tmp1, #RESERVED_SWAPPER_OFFSET	// reserved_pg_dir
 	msr	ttbr0_el1, \tmp1			// set reserved TTBR0_EL1
 	isb
-	add	\tmp1, \tmp1, #PAGE_SIZE
+	add	\tmp1, \tmp1, #RESERVED_SWAPPER_OFFSET
 	msr	ttbr1_el1, \tmp1		// set reserved ASID
 	isb
 	.endm

arch/arm64/include/asm/memory.h

···
 #define IOREMAP_MAX_ORDER	(PMD_SHIFT)
 #endif
 
+/*
+ *  Open-coded (swapper_pg_dir - reserved_pg_dir) as this cannot be calculated
+ *  until link time.
+ */
+#define RESERVED_SWAPPER_OFFSET	(PAGE_SIZE)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/bitops.h>

arch/arm64/include/asm/uaccess.h

···
 	ttbr = read_sysreg(ttbr1_el1);
 	ttbr &= ~TTBR_ASID_MASK;
 	/* reserved_pg_dir placed before swapper_pg_dir */
-	write_sysreg(ttbr - PAGE_SIZE, ttbr0_el1);
+	write_sysreg(ttbr - RESERVED_SWAPPER_OFFSET, ttbr0_el1);
 	isb();
 	/* Set reserved ASID */
 	write_sysreg(ttbr, ttbr1_el1);

arch/arm64/kernel/vmlinux.lds.S

···
  * If padding is applied before .head.text, virt<->phys conversions will fail.
  */
 ASSERT(_text == KIMAGE_VADDR, "HEAD is misaligned")
+
+ASSERT(swapper_pg_dir - reserved_pg_dir == RESERVED_SWAPPER_OFFSET,
+       "RESERVED_SWAPPER_OFFSET is wrong!")