[PULL,10/27] target-arm: Wire up HLT 0xf000 as the A64 semihosting instruction

diff --git a/linux-user/main.c b/linux-user/main.c
index 2c9658e..06dd296 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -1052,6 +1052,9 @@  void cpu_loop(CPUARMState *env)
                 queue_signal(env, info.si_signo, &info);
             }
             break;
+        case EXCP_SEMIHOST:
+            env->xregs[0] = do_arm_semihosting(env);
+            break;
         default:
             fprintf(stderr, "qemu: unhandled CPU exception 0x%x - aborting\n",
                     trapnr);
diff --git a/target-arm/cpu.h b/target-arm/cpu.h
index 0a25335..c794afc 100644
--- a/target-arm/cpu.h
+++ b/target-arm/cpu.h
@@ -56,6 +56,7 @@ 
 #define EXCP_SMC            13   /* Secure Monitor Call */
 #define EXCP_VIRQ           14
 #define EXCP_VFIQ           15
+#define EXCP_SEMIHOST       16   /* semihosting call (A64 only) */
 
 #define ARMV7M_EXCP_RESET   1
 #define ARMV7M_EXCP_NMI     2
diff --git a/target-arm/helper-a64.c b/target-arm/helper-a64.c
index 08c95a3..02fc9b4 100644
--- a/target-arm/helper-a64.c
+++ b/target-arm/helper-a64.c
@@ -514,6 +514,12 @@  void aarch64_cpu_do_interrupt(CPUState *cs)
     case EXCP_VFIQ:
         addr += 0x100;
         break;
+    case EXCP_SEMIHOST:
+        qemu_log_mask(CPU_LOG_INT,
+                      "...handling as semihosting call 0x%" PRIx64 "\n",
+                      env->xregs[0]);
+        env->xregs[0] = do_arm_semihosting(env);
+        return;
     default:
         cpu_abort(cs, "Unhandled exception 0x%x\n", cs->exception_index);
     }
diff --git a/target-arm/internals.h b/target-arm/internals.h
index 924aff9..36a56aa 100644
--- a/target-arm/internals.h
+++ b/target-arm/internals.h
@@ -36,6 +36,7 @@  static inline bool excp_is_internal(int excp)
         || excp == EXCP_HALTED
         || excp == EXCP_EXCEPTION_EXIT
         || excp == EXCP_KERNEL_TRAP
+        || excp == EXCP_SEMIHOST
         || excp == EXCP_STREX;
 }
 
@@ -58,6 +59,7 @@  static const char * const excnames[] = {
     [EXCP_SMC] = "Secure Monitor Call",
     [EXCP_VIRQ] = "Virtual IRQ",
     [EXCP_VFIQ] = "Virtual FIQ",
+    [EXCP_SEMIHOST] = "Semihosting call",
 };
 
 static inline void arm_log_exception(int idx)
diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
index 5c13e15..529bb0c 100644
--- a/target-arm/translate-a64.c
+++ b/target-arm/translate-a64.c
@@ -30,6 +30,7 @@ 
 #include "internals.h"
 #include "qemu/host-utils.h"
 
+#include "exec/semihost.h"
 #include "exec/gen-icount.h"
 
 #include "exec/helper-proto.h"
@@ -1553,8 +1554,27 @@  static void disas_exc(DisasContext *s, uint32_t insn)
             unallocated_encoding(s);
             break;
         }
-        /* HLT */
-        unsupported_encoding(s, insn);
+        /* HLT. This has two purposes.
+         * Architecturally, it is an external halting debug instruction.
+         * Since QEMU doesn't implement external debug, we treat this as
+         * it is required for halting debug disabled: it will UNDEF.
+         * Secondly, "HLT 0xf000" is the A64 semihosting syscall instruction.
+         */
+        if (semihosting_enabled() && imm16 == 0xf000) {
+#ifndef CONFIG_USER_ONLY
+            /* In system mode, don't allow userspace access to semihosting,
+             * to provide some semblance of security (and for consistency
+             * with our 32-bit semihosting).
+             */
+            if (s->current_el == 0) {
+                unsupported_encoding(s, insn);
+                break;
+            }
+#endif
+            gen_exception_internal_insn(s, 0, EXCP_SEMIHOST);
+        } else {
+            unsupported_encoding(s, insn);
+        }
         break;
     case 5:
         if (op2_ll < 1 || op2_ll > 3) {