arm64: early_printk: add support for FastModel console output

Enable early_printk to use the FastModel semihosting to output
the early kernel messages. Works both for host and guest kernels.

To use this feature, pass "early_printk=smh" to the kernel.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/early_printk.c

@@ -40,6 +40,17 @@ static void pl011_printch(char ch)
 		;
 }
 
+/*
+ * Semihosting-based debug console
+ */
+static void smh_printch(char ch)
+{
+	asm volatile("mov  x1, %0\n"
+		     "mov  x0, #3\n"
+		     "hlt  0xf000\n"
+		     : : "r" (&ch) : "x0", "x1", "memory");
+}
+
 struct earlycon_match {
 	const char *name;
 	void (*printch)(char ch);
@@ -47,6 +58,7 @@ struct earlycon_match {
 
 static const struct earlycon_match earlycon_match[] __initconst = {
 	{ .name = "pl011", .printch = pl011_printch, },
+	{ .name = "smh", .printch = smh_printch, },
 	{}
 };