Re: [Qemu-devel] tcg/tcg.c:1892: tcg fatal error

[Igor Kovalenko]

On Wed, Apr 27, 2011 at 12:07 AM, Blue Swirl <address@hidden> wrote:
> On Tue, Apr 26, 2011 at 10:07 PM, Igor Kovalenko
> <address@hidden> wrote:
>> On Tue, Apr 26, 2011 at 10:36 PM, Blue Swirl <address@hidden> wrote:
>>> On Tue, Apr 26, 2011 at 8:02 PM, Artyom Tarasenko <address@hidden> wrote:
>>>> On Mon, Apr 25, 2011 at 10:29 PM, Aurelien Jarno <address@hidden> wrote:
>>>>> On Fri, Apr 22, 2011 at 06:14:06PM +0400, Igor Kovalenko wrote:
>>>>>> On Fri, Apr 22, 2011 at 2:39 AM, Laurent Desnogues
>>>>>> <address@hidden> wrote:
>>>>>> > On Thu, Apr 21, 2011 at 9:45 PM, Igor Kovalenko
>>>>>> > <address@hidden> wrote:
>>>>>> >> On Thu, Apr 21, 2011 at 7:44 PM, Laurent Desnogues
>>>>>> >> <address@hidden> wrote:
>>>>>> >>> On Thu, Apr 21, 2011 at 4:57 PM, Artyom Tarasenko <address@hidden> 
>>>>>> >>> wrote:
>>>>>> >>>> On Tue, Apr 12, 2011 at 4:14 AM, Igor Kovalenko
>>>>>> >>>> <address@hidden> wrote:
>>>>>> >>>>>>> Do you have public test case?
>>>>>> >>>>>>> It is possible to code this delay slot write test but real issue 
>>>>>> >>>>>>> may
>>>>>> >>>>>>> be corruption elsewhere.
>>>>>> >>>>
>>>>>> >>>> The test case is trivial: it's just the two instructions, branch 
>>>>>> >>>> and wrpr.
>>>>>> >>>>
>>>>>> >>>>> In theory there could be multiple issues including compiler 
>>>>>> >>>>> induced ones.
>>>>>> >>>>> I'd prefer to see some kind of reproducible testcase.
>>>>>> >>>>
>>>>>> >>>> Ok, attached a 40 byte long test (the first 32 bytes are not used 
>>>>>> >>>> and
>>>>>> >>>> needed only because the bios entry point is 0x20).
>>>>>> >>>>
>>>>>> >>>> $ git pull && make && sparc64-softmmu/qemu-system-sparc64 -bios
>>>>>> >>>> test-wrpr.bin -nographic
>>>>>> >>>> Already up-to-date.
>>>>>> >>>> make[1]: Nothing to be done for `all'.
>>>>>> >>>> /mnt/terra/projects/vanilla/qemu/tcg/tcg.c:1892: tcg fatal error
>>>>>> >>>> Aborted
>>>>>> >>>
>>>>>> >>> The problem seems to be that wrpr is using a non-local
>>>>>> >>> TCG tmp (cpu_tmp0).
>>>>>> >>
>>>>>> >> Just tried the test case with write to %pil - seems like write itself 
>>>>>> >> is OK.
>>>>>> >> The issue appears to be with save_state() call since adding save_state
>>>>>> >> to %pil case provokes the same tcg abort.
>>>>>> >
>>>>>> > The problem is that cpu_tmp0, not being a local tmp, doesn't
>>>>>> > need to be saved across helper calls.  This results in the
>>>>>> > TCG "optimizer" getting rid of it even though it's later used.
>>>>>> > Look at the log and you'll see what I mean :-)
>>>>>>
>>>>>> I'm not very comfortable with tcg yet. Would it be possible to teach
>>>>>> optimizer working with delay slots? Or do I look in the wrong place.
>>>>>>
>>>>>
>>>>> The problem is not on the TCG side, but on the target-sparc/translate.c
>>>>> side:
>>>>>
>>>>> |                    case 0x32: /* wrwim, V9 wrpr */
>>>>> |                         {
>>>>> |                             if (!supervisor(dc))
>>>>> |                                 goto priv_insn;
>>>>> |                             tcg_gen_xor_tl(cpu_tmp0, cpu_src1, 
>>>>> cpu_src2);
>>>>> | #ifdef TARGET_SPARC64
>>>>>
>>>>> Here cpu_tmp0 is loaded. cpu_tmp0 is a TCG temp, which means it is not
>>>>> saved across TCG branches.
>>>>>
>>>>> [...]
>>>>>
>>>>> |                             case 6: // pstate
>>>>> |                                 save_state(dc, cpu_cond);
>>>>> |                                 gen_helper_wrpstate(cpu_tmp0);
>>>>> |                                 dc->npc = DYNAMIC_PC;
>>>>> |                                 break;
>>>>>
>>>>> save_state() calls save_npc(), which in turns might call
>>>>> gen_generic_branch():
>>>>
>>>> Hmm. This is not the only instruction using save_state() and cpu_tmp0.
>>>> At least ldd is another example.
>>>>
>>>>> | static inline void gen_generic_branch(target_ulong npc1, target_ulong 
>>>>> npc2,
>>>>> |                                       TCGv r_cond)
>>>>> | {
>>>>> |     int l1, l2;
>>>>> |
>>>>> |     l1 = gen_new_label();
>>>>> |     l2 = gen_new_label();
>>>>> |
>>>>> |     tcg_gen_brcondi_tl(TCG_COND_EQ, r_cond, 0, l1);
>>>>> |
>>>>> |     tcg_gen_movi_tl(cpu_npc, npc1);
>>>>> |     tcg_gen_br(l2);
>>>>> |
>>>>> |     gen_set_label(l1);
>>>>> |     tcg_gen_movi_tl(cpu_npc, npc2);
>>>>> |     gen_set_label(l2);
>>>>> | }
>>>>>
>>>>> And here is the TCG branch, which drop the TCG temp cpu_temp0.
>>>>>
>>>>> The solution is either to rewrite gen_generic_branch() without TCG
>>>>> branches, or to use a TCG temp local instead of a TCG temp.
>>>>
>>>> You mean something like
>>>>
>>>>                            case 6: // pstate
>>>>                                {
>>>>                                    TCGv r_temp;
>>>>
>>>>                                    r_temp = tcg_temp_new();
>>>>                                    tcg_gen_mov_tl(r_temp, cpu_tmp0);
>>>>                                    save_state(dc, cpu_cond);
>>>>                                    gen_helper_wrpstate(r_temp);
>>>>                                    tcg_temp_free(r_temp);
>>>>                                    dc->npc = DYNAMIC_PC;
>>>>                                }
>>>>                                break;
>>>>
>>>> ?
>>>> This fails with the same error message.
>>>
>>> Close, but you need to use tcg_temp_local_new(). Does this work?
>>>
>>> diff --git a/target-sparc/translate.c b/target-sparc/translate.c
>>> index 3c958b2..52fa2f1 100644
>>> --- a/target-sparc/translate.c
>>> +++ b/target-sparc/translate.c
>>> @@ -3505,9 +3505,15 @@ static void disas_sparc_insn(DisasContext * dc)
>>>                                 tcg_gen_mov_tl(cpu_tbr, cpu_tmp0);
>>>                                 break;
>>>                             case 6: // pstate
>>> -                                save_state(dc, cpu_cond);
>>> -                                gen_helper_wrpstate(cpu_tmp0);
>>> -                                dc->npc = DYNAMIC_PC;
>>> +                                {
>>> +                                    TCGv r_tmp = tcg_temp_local_new();
>>> +
>>> +                                    tcg_gen_mov_tl(r_tmp, cpu_tmp0);
>>> +                                    save_state(dc, cpu_cond);
>>> +                                    gen_helper_wrpstate(r_tmp);
>>> +                                    tcg_temp_free_ptr(r_tmp);
>>> +                                    dc->npc = DYNAMIC_PC;
>>> +                                }
>>>                                 break;
>>>                             case 7: // tl
>>>                                 save_state(dc, cpu_cond);
>>>
>>
>> This one works (I have tested similar change but tcg_temp_free() would
>> be OK as well)
>
> Thanks for spotting, that was not intentional.
>
>> Could anyone please explain why it still works without being in delay
>> slot? Simply pointing at optimizer does not work :(
>
> save_npc() will call gen_generic_branch() only if dc->npc == JUMP_PC,
> which is used for delay slot handling. Otherwise gen_generic_branch()
> is not called and thus  brcond does not get used, so the temporary
> will not be flushed for non-delay slot cases.

Great that explains it all, thanks! I think audit over translate.c
would be needed, alternatively all main switch temps for sparc64 could
be made local as hinted by Laurent - provided there is no significant
speed impact.

-- 
Kind regards,
Igor V. Kovalenko