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

[Aurelien Jarno <aurelien@aurel32.net>]

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.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net