baremetal: arm multicore attempt

baremetal/arch/aarch64/no_bootloader/multicore.S

@@ -0,0 +1,35 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-multicore */
+
+.global mystart
+mystart:
+    /* Reset spinlock. */
+    mov x0, #0
+    ldr x1, =spinlock
+    str x0, [x1]
+
+    /* Read cpu id into x1. */
+    mrs x1, mpidr_el1
+    and x1, x1, #3
+    cbz x1, 1f
+    /* Only CPU 1 reaches this point and sets the spinlock. */
+    mov x0, #1
+    ldr x1, =spinlock
+    str x0, [x1]
+    b .
+1:
+    /* Only CPU 0 reaches this point. */
+    ldr x0, spinlock
+    cbz x0, 1b
+
+    /* Semihost exit. */
+    mov x1, #0x26
+    movk x1, #2, lsl #16
+    str x1, [sp,#0]
+    mov x0, #0
+    str x0, [sp,#8]
+    mov x1, sp
+    mov w0, #0x18
+    hlt 0xf000
+
+spinlock:
+    .skip 8

baremetal/arch/aarch64/no_bootloader/semihost_exit.S

@@ -1,3 +1,5 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#semihosting */
+
 .global mystart
 mystart:
     mov x1, #0x26