ppc: fix crash during branch stepping

The PPC BRANCH exception could bubble up, but this is an QEMU internal exception
and QEMU then crased. Instead it should trigger TRACE exception, according to
PPC 2.07 book. It could happen only when using branch stepping, which is not
commonly used.

Change gen_prep_dbgex do do trigger TRACE. The excp, argument is now removed,
since the type of exception can be inferred from the singlestep_enabled flags.
removed the guards around gen_exception, since they are unnecessary.

Fixes: 0e3bf48 ("ppc: add DBCR based debugging").
Signed-off-by: Roman Kapl <rka@sysgo.com>
Message-Id: <20190212121255.2279-1-rka@sysgo.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/translate.c

@@ -287,26 +287,22 @@ static void gen_exception_nip(DisasContext *ctx, uint32_t excp,
     ctx->exception = (excp);
 }
 
-/* Translates the EXCP_TRACE/BRANCH exceptions used on most PowerPCs to
- * EXCP_DEBUG, if we are running on cores using the debug enable bit (e.g.
- * BookE).
+/*
+ * Tells the caller what is the appropriate exception to generate and prepares
+ * SPR registers for this exception.
+ *
+ * The exception can be either POWERPC_EXCP_TRACE (on most PowerPCs) or
+ * POWERPC_EXCP_DEBUG (on BookE).
  */
-static uint32_t gen_prep_dbgex(DisasContext *ctx, uint32_t excp)
+static uint32_t gen_prep_dbgex(DisasContext *ctx)
 {
-    if ((ctx->singlestep_enabled & CPU_SINGLE_STEP)
-        && (excp == POWERPC_EXCP_BRANCH)) {
-        /* Trace excpt. has priority */
-        excp = POWERPC_EXCP_TRACE;
-    }
     if (ctx->flags & POWERPC_FLAG_DE) {
         target_ulong dbsr = 0;
-        switch (excp) {
-        case POWERPC_EXCP_TRACE:
+        if (ctx->singlestep_enabled & CPU_SINGLE_STEP) {
             dbsr = DBCR0_ICMP;
-            break;
-        case POWERPC_EXCP_BRANCH:
+        } else {
+            /* Must have been branch */
             dbsr = DBCR0_BRT;
-            break;
         }
         TCGv t0 = tcg_temp_new();
         gen_load_spr(t0, SPR_BOOKE_DBSR);
@@ -315,7 +311,7 @@ static uint32_t gen_prep_dbgex(DisasContext *ctx, uint32_t excp)
         tcg_temp_free(t0);
         return POWERPC_EXCP_DEBUG;
     } else {
-        return excp;
+        return POWERPC_EXCP_TRACE;
     }
 }
 
@@ -3652,10 +3648,8 @@ static void gen_lookup_and_goto_ptr(DisasContext *ctx)
         if (sse & GDBSTUB_SINGLE_STEP) {
             gen_debug_exception(ctx);
         } else if (sse & (CPU_SINGLE_STEP | CPU_BRANCH_STEP)) {
-            uint32_t excp = gen_prep_dbgex(ctx, POWERPC_EXCP_BRANCH);
-            if (excp != POWERPC_EXCP_NONE) {
-                gen_exception(ctx, excp);
-            }
+            uint32_t excp = gen_prep_dbgex(ctx);
+            gen_exception(ctx, excp);
         }
         tcg_gen_exit_tb(NULL, 0);
     } else {
@@ -7790,9 +7784,8 @@ static void ppc_tr_translate_insn(DisasContextBase *dcbase, CPUState *cs)
                  ctx->exception != POWERPC_SYSCALL &&
                  ctx->exception != POWERPC_EXCP_TRAP &&
                  ctx->exception != POWERPC_EXCP_BRANCH)) {
-        uint32_t excp = gen_prep_dbgex(ctx, POWERPC_EXCP_TRACE);
-        if (excp != POWERPC_EXCP_NONE)
-            gen_exception_nip(ctx, excp, ctx->base.pc_next);
+        uint32_t excp = gen_prep_dbgex(ctx);
+        gen_exception_nip(ctx, excp, ctx->base.pc_next);
     }
 
     if (tcg_check_temp_count()) {