[Qemu-devel] Re: sparc32 FPU SP Invalid CEXC Test

[Artyom Tarasenko]

2010/4/16 Artyom Tarasenko <address@hidden>:
> 2010/4/15 Artyom Tarasenko <address@hidden>:
>> 2010/4/15 Blue Swirl <address@hidden>:
>>> On 4/15/10, Artyom Tarasenko <address@hidden> wrote:
>>>> 2010/4/15 Artyom Tarasenko <address@hidden>:
>>>>
>>>> > One of LX's tests crashes pretty hard, causing qemu abort.
>>>>  > I've tried to look how does the execution flow works with -d in_asm.
>>>>  > Does the address in the log show the guest's PC register?
>>>>
>>>>
>>>> It's probably sort of a "timing" issue.
>>>>
>>>>  Can we check exceptions not just on jumps, but also on floating poit
>>>>  operations which may cause a trap?
>>>>  These traps are supposed to be syncronous.
>>>
>>> Yes, the bug is that PC and NPC are not saved before executing FPU
>>> instructions. Please try this patch.
>>
>> The patch gets it a couple of tests further:
>>
>> FPU SP Invalid CEXC Test
>> FPU SP Overflow CEXC Test
>> FPU SP Divide-by-0 CEXC Test
>> FPU SP Inexact CEXC Test
>> FPU SP Trap Priority >  Test Unassigned mem write access of 4 bytes to
>> 000000008421f000 from 700030f8
>>
>> FPU SP Trap Priority <  Test
>>     ERROR : Unexpected Synchronous Trap Taken, Trap Type = 00000008,
>> PSR = 414010c4, PC = 70003190, TBR = 00000080
>>     STATUS : Entering scope loop .... Press <A> key to Abort!qemu:
>> fatal: Trap 0x03 while interrupts disabled, Error state
>> pc: 0000217c  npc: 00003170
>> General Registers:
>> %g0-7: 00000000 00003170 00000055 00000001 00000002 00000000 00000000 
>> 00000000
>>
>> Current Register Window:
>> %o0-7: 00000000 00000999 00000000 00000000 00000000 00000000 0001fba0 
>> 7000971c
>> %l0-7: 0002fff8 00000000 00000000 00000000 00000000 ffffffff 00000000 
>> 00000000
>> %i0-7: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
>> 00000000
>>
>> Floating Point Registers:
>> %f00: 000000002.890625 000000025.000000 000000000.000000 000000000.000000
>> %f04: 000000002.890625 000000000.000000 000000002.890625 000000000.000000
>> %f08: 000000003.390625 000000000.000000 000000002.250000 000000000.000000
>> %f12: 000000002.890625 000000000.000000 000000002.312500 000000000.000000
>> %f16: 000000002.312500 000000000.000000 000000002.890625 000000000.000000
>> %f20: 000000002.718750 000000000.000000 000000002.562500 000000000.000000
>> %f24: 000000002.890625 000000000.000000 000000002.968750 000000000.000000
>> %f28: 000000002.312500 000000000.000000 000000002.890625 000000000.000000
>> psr: 41000000 (icc: ---- SPE: ---) wim: 00000002
>> fsr: 0f884002 y: 00000000
>> Aborted
>>
>>
>> The code:
>>
>>   0x70003174:  sethi  %hi(0x41c80000), %l3
>>   0x70003178:  add  %l4, 2, %l5
>>   0x7000317c:  st  %l3, [ %l4 ]
>>   0x70003180:  ld  [ %l4 ], %f1
>>   0x70003184:  clr  [ %l4 ]
>>   0x70003188:  ld  [ %l4 ], %f2
>>   0x7000318c:  mov  7, %g5
>>   0x70003190:  fdivs  %f1, %f2, %f3
>
> And what is even more strange it looks in qemu.log like if trap is taken,
> gdb doesn't stop at the 0x080 breakpoint after this operation.
> Whether I do a stepi or nexti, it just continues up to the crash.
> Let me know if I can provide more information.
>
> Breakpoint 2, 0x00000080 in ?? ()
> (gdb) cont
> Continuing.
>
> Breakpoint 6, 0x70003190 in ?? ()
> (gdb) stepi
> Remote connection closed
> (gdb)

The trick was not to set the breakpoint at 0x70003190. Then the
breakpoint at 0x80 works.
And I think I found a hint:

http://www.cmpe.boun.edu.tr/courses/cmpe511/fall2004/Ozan Aktan -
Supersparc Architecture.doc

"One unique feature of the floating-point unit is that dependent
floating-point instructions may be issued in the same instruction
group as the dependent floating-point operation. As an example, the
following instructions can issue in a single clock cycle:
LDD                   [%i0 + %i1], %f2
FMULD              %f2, %f4, %f6   "

We also have a dependent instructions
0x700030f4:  fdivs  %f1, %f2, %f3
0x700030f8:  st  %f3, [ %l6 ]

which must produce two traps simultaneously: division by zero and
unaligned access. Unaligned access is a higher priority trap, so it
must be processed first.

In the previous test (which passed) the store produces a data access
exception which has lower priority than division by zero. The test
passes because it is bad.

-- 
Regards,
Artyom Tarasenko

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