arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

According to the arm64 boot protocol, registers x1 to x3 should be
zero upon kernel entry, and non-zero values are reserved for future
use. This future use is going to be problematic if we never enforce
the current rules, so start enforcing them now, by emitting a warning
if non-zero values are detected.
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/head.S

@@ -233,7 +233,7 @@ section_table:
 #endif
 
 ENTRY(stext)
-	mov	x21, x0				// x21=FDT
+	bl	preserve_boot_args
 	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
 	adrp	x24, __PHYS_OFFSET
 	bl	set_cpu_boot_mode_flag
@@ -252,6 +252,23 @@ ENTRY(stext)
 	b	__cpu_setup			// initialise processor
 ENDPROC(stext)
 
+/*
+ * Preserve the arguments passed by the bootloader in x0 .. x3
+ */
+preserve_boot_args:
+	mov	x21, x0				// x21=FDT
+
+	adr_l	x0, boot_args			// record the contents of
+	stp	x21, x1, [x0]			// x0 .. x3 at kernel entry
+	stp	x2, x3, [x0, #16]
+
+	dmb	sy				// needed before dc ivac with
+						// MMU off
+
+	add	x1, x0, #0x20			// 4 x 8 bytes
+	b	__inval_cache_range		// tail call
+ENDPROC(preserve_boot_args)
+
 /*
  * Determine validity of the x21 FDT pointer.
  * The dtb must be 8-byte aligned and live in the first 512M of memory.

arch/arm64/kernel/setup.c

@@ -115,6 +115,11 @@ void __init early_print(const char *str, ...)
 	printk("%s", buf);
 }
 
+/*
+ * The recorded values of x0 .. x3 upon kernel entry.
+ */
+u64 __cacheline_aligned boot_args[4];
+
 void __init smp_setup_processor_id(void)
 {
 	u64 mpidr = read_cpuid_mpidr() & MPIDR_HWID_BITMASK;
@@ -412,6 +417,12 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;
 #endif
 #endif
+	if (boot_args[1] || boot_args[2] || boot_args[3]) {
+		pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"
+			"\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n"
+			"This indicates a broken bootloader or old kernel\n",
+			boot_args[1], boot_args[2], boot_args[3]);
+	}
 }
 
 static int __init arm64_device_init(void)