Re: [Qemu-devel] [PATCH] Regression for m68k causing Single-Step via GDB/RSP to not single step

[Richard Henderson]

On 6/18/19 11:44 AM, Laurent Vivier wrote:
> Le 26/05/2019 à 09:50, Lucien Murray-Pitts a écrit :
>> A regression that was introduced, with the refactor to TranslatorOps,
>> drops two lines that update the PC when single-stepping is being performed.
>> ( short commit 11ab74b )
>>
>> This patch resolves that issue.
> 
> Fixes: 11ab74b01e0a ("target/m68k: Convert to TranslatorOps")
> 
>> Signed-off-by: Lucien Murray-Pitts <address@hidden>
>> ---
>>  target/m68k/translate.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/target/m68k/translate.c b/target/m68k/translate.c
>> index f0534a4ba0..2922ea79c3 100644
>> --- a/target/m68k/translate.c
>> +++ b/target/m68k/translate.c
>> @@ -6130,6 +6130,8 @@ static void m68k_tr_tb_stop(DisasContextBase *dcbase, 
>> CPUState *cpu)
>>          return;
>>      }
>>      if (dc->base.singlestep_enabled) {
>> +        update_cc_op(dc);
>> +        tcg_gen_movi_i32(QREG_PC, dc->pc);
>>          gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
>>          return;
>>      }
>>
> 
> I've tested this fix single-stepping on a kernel, these two lines are 
> not enough to fix the problem. In fact four lines have been dropped and 
> we must re-add them all:
> 
> iff --git a/target/m68k/translate.c b/target/m68k/translate.c
> index d0f6d1f5cc..6c78001501 100644
> --- a/target/m68k/translate.c
> +++ b/target/m68k/translate.c
> @@ -6200,6 +6200,10 @@ static void m68k_tr_tb_stop(DisasContextBase *dcbase, 
> CPUState *cpu)
>          return;
>      }
>      if (dc->base.singlestep_enabled) {
> +        if (dc->base.is_jmp != DISAS_JUMP) {
> +            update_cc_op(dc);
> +            tcg_gen_movi_i32(QREG_PC, dc->pc);
> +        }
>          gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
>          return;
>      }

Even this isn't quite right, according to the comments in the switch that
follows.  I think it'd be best written like so.


r~


diff --git a/target/m68k/translate.c b/target/m68k/translate.c
index 2ae537461f..b61c7ea0f1 100644
--- a/target/m68k/translate.c
+++ b/target/m68k/translate.c
@@ -6124,27 +6124,34 @@ static void m68k_tr_tb_stop(DisasContextBase *dcbase,
CPUState *cpu)
 {
     DisasContext *dc = container_of(dcbase, DisasContext, base);

-    if (dc->base.is_jmp == DISAS_NORETURN) {
-        return;
-    }
-    if (dc->base.singlestep_enabled) {
-        gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
-        return;
-    }
-
     switch (dc->base.is_jmp) {
+    case DISAS_NORETURN:
+        break;
     case DISAS_TOO_MANY:
         update_cc_op(dc);
-        gen_jmp_tb(dc, 0, dc->pc);
+        if (dc->base.singlestep_enabled) {
+            tcg_gen_movi_i32(QREG_PC, dc->pc);
+            gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
+        } else {
+            gen_jmp_tb(dc, 0, dc->pc);
+        }
         break;
     case DISAS_JUMP:
         /* We updated CC_OP and PC in gen_jmp/gen_jmp_im.  */
-        tcg_gen_lookup_and_goto_ptr();
+        if (dc->base.singlestep_enabled) {
+            gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
+        } else {
+            tcg_gen_lookup_and_goto_ptr();
+        }
         break;
     case DISAS_EXIT:
         /* We updated CC_OP and PC in gen_exit_tb, but also modified
            other state that may require returning to the main loop.  */
-        tcg_gen_exit_tb(NULL, 0);
+        if (dc->base.singlestep_enabled) {
+            gen_helper_raise_exception(cpu_env, tcg_const_i32(EXCP_DEBUG));
+        } else {
+            tcg_gen_exit_tb(NULL, 0);
+        }
         break;
     default:
         g_assert_not_reached();