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

[Igor Kovalenko <igor.v.kovalenko@gmail.com>]

On Tue, Apr 26, 2011 at 12:29 AM, Aurelien Jarno <aurelien@aurel32.net> 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
>> <laurent.desnogues@gmail.com> wrote:
>> > On Thu, Apr 21, 2011 at 9:45 PM, Igor Kovalenko
>> > <igor.v.kovalenko@gmail.com> wrote:
>> >> On Thu, Apr 21, 2011 at 7:44 PM, Laurent Desnogues
>> >> <laurent.desnogues@gmail.com> wrote:
>> >>> On Thu, Apr 21, 2011 at 4:57 PM, Artyom Tarasenko <atar4qemu@gmail.com> wrote:
>> >>>> On Tue, Apr 12, 2011 at 4:14 AM, Igor Kovalenko
>> >>>> <igor.v.kovalenko@gmail.com> 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.
I looked at my linux kernel builds and do not see any wrpr pstate in delay slot.

-- 
Kind regards,
Igor V. Kovalenko