target/m68k: Perform the semihosting test during translate

Replace EXCP_HALT_INSN by EXCP_SEMIHOSTING.  Perform the pre-
and post-insn tests during translate, leaving only the actual
semihosting operation for the exception.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/m68k/cpu.h

@@ -66,7 +66,7 @@
 #define EXCP_MMU_ACCESS     58  /* MMU Access Level Violation Error */
 
 #define EXCP_RTE            0x100
-#define EXCP_HALT_INSN      0x101
+#define EXCP_SEMIHOSTING    0x101
 
 #define M68K_DTTR0   0
 #define M68K_DTTR1   1

target/m68k/op_helper.c

@@ -202,18 +202,8 @@ static void cf_interrupt_all(CPUM68KState *env, int is_hw)
             /* Return from an exception.  */
             cf_rte(env);
             return;
-        case EXCP_HALT_INSN:
-            if (semihosting_enabled((env->sr & SR_S) == 0)
-                    && (env->pc & 3) == 0
-                    && cpu_lduw_code(env, env->pc - 4) == 0x4e71
-                    && cpu_ldl_code(env, env->pc) == 0x4e7bf000) {
-                env->pc += 4;
-                do_m68k_semihosting(env, env->dregs[0]);
-                return;
-            }
-            cs->halted = 1;
-            cs->exception_index = EXCP_HLT;
-            cpu_loop_exit(cs);
+        case EXCP_SEMIHOSTING:
+            do_m68k_semihosting(env, env->dregs[0]);
             return;
         }
     }

target/m68k/translate.c

@@ -26,12 +26,11 @@
 #include "qemu/log.h"
 #include "qemu/qemu-print.h"
 #include "exec/translator.h"
-
 #include "exec/helper-proto.h"
 #include "exec/helper-gen.h"
-
 #include "exec/log.h"
 #include "fpu/softfloat.h"
+#include "semihosting/semihost.h"
 
 #define HELPER_H "helper.h"
 #include "exec/helper-info.c.inc"
@@ -1401,6 +1400,40 @@ static void gen_jmp_tb(DisasContext *s, int n, target_ulong dest,
     s->base.is_jmp = DISAS_NORETURN;
 }
 
+#ifndef CONFIG_USER_ONLY
+static bool semihosting_test(DisasContext *s)
+{
+    uint32_t test;
+
+    if (!semihosting_enabled(IS_USER(s))) {
+        return false;
+    }
+
+    /*
+     * "The semihosting instruction is immediately preceded by a
+     * nop aligned to a 4-byte boundary..."
+     * The preceding 2-byte (aligned) nop plus the 2-byte halt/bkpt
+     * means that we have advanced 4 bytes from the required nop.
+     */
+    if (s->pc % 4 != 0) {
+        return false;
+    }
+    test = translator_lduw(s->env, &s->base, s->pc - 4);
+    if (test != 0x4e71) {
+        return false;
+    }
+    /* "... and followed by an invalid sentinel instruction movec %sp,0." */
+    test = translator_ldl(s->env, &s->base, s->pc);
+    if (test != 0x4e7bf000) {
+        return false;
+    }
+
+    /* Consume the sentinel. */
+    s->pc += 4;
+    return true;
+}
+#endif /* !CONFIG_USER_ONLY */
+
 DISAS_INSN(scc)
 {
     DisasCompare c;
@@ -4465,8 +4498,12 @@ DISAS_INSN(halt)
         gen_exception(s, s->base.pc_next, EXCP_PRIVILEGE);
         return;
     }
-
-    gen_exception(s, s->pc, EXCP_HALT_INSN);
+    if (semihosting_test(s)) {
+        gen_exception(s, s->pc, EXCP_SEMIHOSTING);
+        return;
+    }
+    tcg_gen_movi_i32(cpu_halted, 1);
+    gen_exception(s, s->pc, EXCP_HLT);
 }
 
 DISAS_INSN(stop)