baremetal aarch64: create C version of multicore.S as well

Attempted to do the same for arm, but it failed.

baremetal/lib/aarch64.S

@@ -2,6 +2,11 @@
 
 .global lkmc_start
 lkmc_start:
+    /* Make all CPUs except CPU0 sleep by default. */
+    mrs x0, mpidr_el1
+    ands x0, x0, 3
+    bne lkmc_cpu_not_0
+
     /* Load the vector table. */
     ldr x0, =lkmc_vector_table
     msr vbar_el1, x0
@@ -41,3 +46,8 @@ LKMC_WEAK(lkmc_vector_trap_handler)
     bl abort
 lkmc_vector_trap_handler_error_message:
     .asciz "error: unexpected interrupt"
+
+/* Default action for CPUs besides the first one: sleep forever. */
+LKMC_WEAK(lkmc_cpu_not_0)
+    wfe
+    b lkmc_cpu_not_0

baremetal/lib/arm.S

@@ -2,6 +2,11 @@
 
 .global lkmc_start
 lkmc_start:
+    /* Make all CPUs except CPU0 sleep by default. */
+    mrc p15, 0, r0, c0, c0, 5
+    ands r0, r0, 3
+    bne lkmc_cpu_not_0
+
     /* Prepare the stack for main, mandatory for C code. */
     ldr sp, =stack_top
 
@@ -28,3 +33,8 @@ lkmc_start:
 
     /* If main returns, exit. */
     bl exit
+
+/* Default action for CPUs besides the first one: sleep forever. */
+LKMC_WEAK(lkmc_cpu_not_0)
+    wfe
+    b lkmc_cpu_not_0

baremetal/lib/syscalls.c

@@ -3,14 +3,6 @@
 #include <sys/stat.h>
 
 #include <lkmc.h>
-#include <lkmc/m5ops.h>
-
-void lkmc_baremetal_on_exit_callback(int status, void *arg) {
-    (void)arg;
-    if (status != 0) {
-        printf("lkmc_exit_status_%d\n", status);
-    }
-}
 
 enum {
     UART_FR_RXFE = 0x10,
@@ -19,44 +11,18 @@ enum {
 #define UART_DR(baseaddr) (*(unsigned int *)(baseaddr))
 #define UART_FR(baseaddr) (*(((unsigned int *)(baseaddr))+6))
 
+void lkmc_baremetal_on_exit_callback(int status, void *arg) {
+    (void)arg;
+    if (status != 0) {
+        printf("lkmc_exit_status_%d\n", status);
+    }
+}
+
 int _close(int file) {
     LKMC_UNUSED(file);
     return -1;
 }
 
-void _exit(int status) {
-    LKMC_UNUSED(status);
-#if LKMC_GEM5
-    LKMC_M5OPS_EXIT;
-#else
-#if defined(__arm__)
-    __asm__ __volatile__ (
-        "mov r0, #0x18\n"
-        "ldr r1, =#0x20026\n"
-        "svc 0x00123456\n"
-        :
-        :
-        : "r0", "r1"
-    );
-#elif defined(__aarch64__)
-    /* TODO actually use the exit value here, just for fun. */
-    __asm__ __volatile__ (
-        "mov x1, #0x26\n" \
-        "movk x1, #2, lsl #16\n" \
-        "str x1, [sp,#0]\n" \
-        "mov x0, #0\n" \
-        "str x0, [sp,#8]\n" \
-        "mov x1, sp\n" \
-        "mov w0, #0x18\n" \
-        "hlt 0xf000\n"
-        :
-        :
-        : "x0", "x1"
-    );
-#endif
-#endif
-}
-
 int _fstat(int file, struct stat *st) {
     LKMC_UNUSED(file);
     st->st_mode = S_IFCHR;

baremetal/lib/syscalls_asm.S

@@ -0,0 +1,35 @@
+#include <lkmc.h>
+
+/* This is implemented in assembly so that it does not use the stack,
+ * and thus can be called safely from programs without the bootloader.
+ * C signature:
+ *
+ * void _exit(int status)
+ *
+ * If only there was a GCC attribute to create such a function!
+ */
+.text
+.global _exit
+_exit:
+#if LKMC_GEM5
+    LKMC_M5OPS_EXIT_ASM
+#else
+/* Use semihosting:
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#semihosting */
+#if defined(__arm__)
+    mov r0, #0x18
+    ldr r1, =#0x20026
+    svc 0x00123456
+#elif defined(__aarch64__)
+    mov x1, 0x26
+    movk x1, 2, lsl 16
+    ldr x2, =.Lsemihost_args
+    str x1, [x2, 0]
+    str x0, [x2, 8]
+    mov x1, x2
+    mov w0, 0x18
+    hlt 0xf000
+.Lsemihost_args:
+    .skip 16
+#endif
+#endif