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

[Artyom Tarasenko]

On Tue, Apr 26, 2011 at 5:34 AM, Igor Kovalenko
<address@hidden> wrote:
> On Tue, Apr 26, 2011 at 12:29 AM, 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():
>>
>> | 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.
>
> Thanks!
>
> I think the issue is more clear now, and loading to local temporary
> works in this case.
> Does not explain why unmodified qemu works with wrpr pstate not in delay slot.

Because the TCG branch is not generated in save_npc()?

> I looked at my linux kernel builds and do not see any wrpr pstate in delay 
> slot.

Meaning you are not going to fix the bug? ;-)


-- 
Regards,
Artyom Tarasenko

solaris/sparc under qemu blog: http://tyom.blogspot.com/