baremetal: arm multicore attempt

Author: cirosantilli

README.adoc

@@ -10419,7 +10419,13 @@ help architecture
 
 shows ARM version up to `armv6`, so maybe `armv6` is not implemented?
 
-=== ARM exception level
+=== ARM baremetal
+
+In this section we will focus on learning ARM architecture concepts that can only learnt on baremetal setups.
+
+Userland information can be found at: https://github.com/cirosantilli/arm-assembly-cheat
+
+==== ARM exception level
 
 ARM exception levels are analogous to x86 <<ring0,rings>>.
 
@@ -10491,6 +10497,25 @@ output:
 3
 ....
 
+==== ARM multicore
+
+TODO get working: CPU1 not waking up:
+
+....
+./run --arch aarch64 --baremetal arch/aarch64/no_bootloader/multicore
+....
+
+Source: link:baremetal/arch/aarch64/no_bootloader/multicore.S[]
+
+CPU 0 of this program enters a spinlock loop: it repeatedly checks if a given memory address is `1`.
+
+So, we need CPU 1 to come to the rescue to that memory address be `1`, otherwise CPU 0 will be stuck there forever.
+
+Bibliography:
+
+* https://stackoverflow.com/questions/20055754/arm-start-wakeup-bringup-the-other-cpu-cores-aps-and-pass-execution-start-addre
+* https://stackoverflow.com/questions/980999/what-does-multicore-assembly-language-look-like/33651438#33651438
+
 === How we got some baremetal stuff to work
 
 It is nice when thing just work.

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