Re: [Qemu-devel] [Qemu-ppc] Help needed testing on ppc

[Tom Musta]

On 6/17/2014 6:05 AM, BALATON Zoltan wrote:
> On Tue, 17 Jun 2014, Alexander Graf wrote:
>>>>> http://goliat.eik.bme.hu/~balaton/oftest/results/
>>>>>
>>>>> The results show that the stwx instruction with reserved bit set does not 
>>>>> change status bits and does not generate an exception on any CPU tested 
>>>>> (G3 and G4) so it is most probably just ignored as we thought.
>>>>
>>>> [adding qemu-ppc and tom to CC]
>>>>
>>>> Tom already commented on this. Is there a pattern that matches all the 
>>>> indexed load/store instructions or is stwx a one-off?
>>>
>>> Is this a question to whom? If to me I don't understand it.
>>
>> stwx is part of a group of instructions. It's very rare that hardware only 
>> shows certain behavior (like ignore a reserved bit) for single instructions. 
>> Usually it happens on complete groups.
> 
> Who would know that? The test only tested stwx and I assume the same that 
> this should not behave differently than any other instruction. Also this is 
> the only instruction that was used with the set reserved bit in MorphOS and 
> the patch ignoring reserved
> bits for this group of instructions that we discussed earlier seems to fix 
> it. (There's another case with an altivec instruction with a similar failure 
> but I did not look at that yet if that's a reserved bit too or something 
> else.) As to why it's in
> MorphOS I don't know, I got no answer from them.
> 
> Regards,
> BALATON Zoltan

I am looking at the test case source code and do not see how you are setting 
the reserved bit.  Maybe I am missing some cleverness in how the test is built?

#include <prom.h>

void stwxtest(void)
{
  unsigned int val, cr, cr1, cr2;

  prom_printf("Testing stwx reserved bit\n");

  val = 0;

  asm volatile("mfcr %0               \n\t"
               "bl 1f                 \n\t"
               "mfcr %1               \n\t"
               "mflr 10               \n\t"
               "lwz %0, 36(10)        \n\t"
               "ori %0, %0, 1         \n\t"
               "stw %0, 36(10)        \n\t"
               "mfcr %0               \n\t"
               "bl 1f                 \n\t"
               "mfcr %2               \n\t"
               "b 2f                  \n\t"
               "1: stwx %0, %4, %6    \n\t"          <<<<<<<<<<<<< just a 
normal stwx, right?
               "blr                   \n\t"
               "2:                    \n\t"
                : "=&r"(cr), "=&r"(cr1), "=&r"(cr2), "=m"(val)
                : "r"(&val), "m"(val), "r"(8)
                : "r8", "r9", "r10", "cc", "memory");

  prom_printf("old cr  (mem):\t%#x\n", val);
  prom_printf("old cr  (reg):\t%#x\n", cr);
  prom_printf("new cr1 (reg):\t%#x\n", cr1);
  prom_printf("new cr2 (reg):\t%#x\n", cr2);
}


But the objdump of your test binary does not show that it is set either:

00102784 <stwxtest>:
stwxtest():
  102784:       7c 08 02 a6     mflr    r0
  102788:       94 21 ff d0     stwu    r1,-48(r1)
  10278c:       3c 60 00 10     lis     r3,16
  102790:       38 63 32 29     addi    r3,r3,12841
  102794:       bf a1 00 24     stmw    r29,36(r1)
  102798:       90 01 00 34     stw     r0,52(r1)
  10279c:       4c c6 31 82     crclr   4*cr1+eq
  1027a0:       4b ff e2 75     bl      100a14 <prom_printf>
  1027a4:       7c 27 0b 78     mr      r7,r1
  1027a8:       39 20 00 00     li      r9,0
  1027ac:       95 27 00 08     stwu    r9,8(r7)
  1027b0:       38 c0 00 08     li      r6,8
  1027b4:       7f a0 00 26     mfcr    r29
  1027b8:       48 00 00 29     bl      1027e0 <stwxtest+0x5c>
  1027bc:       7f c0 00 26     mfcr    r30
  1027c0:       7d 48 02 a6     mflr    r10
  1027c4:       83 aa 00 24     lwz     r29,36(r10)
  1027c8:       63 bd 00 01     ori     r29,r29,1
  1027cc:       93 aa 00 24     stw     r29,36(r10)
  1027d0:       7f a0 00 26     mfcr    r29
  1027d4:       48 00 00 0d     bl      1027e0 <stwxtest+0x5c>
  1027d8:       7f e0 00 26     mfcr    r31
  1027dc:       48 00 00 0c     b       1027e8 <stwxtest+0x64>
  1027e0:       7f a7 31 2e     stwx    r29,r7,r6  
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  1027e4:       4e 80 00 20     blr