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Re: [Qemu-ppc] [OpenBIOS] Running client with MMU off

From: BALATON Zoltan
Subject: Re: [Qemu-ppc] [OpenBIOS] Running client with MMU off
Date: Thu, 26 Jun 2014 13:10:19 +0200 (CEST)
User-agent: Alpine 2.02 (LMD 1266 2009-07-14)

On Thu, 26 Jun 2014, Alexander Graf wrote:
On 26.06.14 01:36, BALATON Zoltan wrote:
On Wed, 25 Jun 2014, Alexander Graf wrote:
On 25.06.14 12:40, BALATON Zoltan wrote:
On Wed, 25 Jun 2014, BALATON Zoltan wrote:
ppc_store_sdr1: 0fe00000
helper_store_sr: reg=0 20000400 00000000
helper_store_sr: reg=0 00000000 20000400
Raise exception at 0041cd00 => 00000003 (40000000)

^^^ This exception should not happen. It is trying to handle it but the handlers are not working yet and gets in an infinite loop. It boots if MMU is disabled while this part runs but MorphOS does not disable it yet and according to my oftest results they are enabled on Apple too. How does it work on real hardware and why does it fail on QEMU? (Note the the value of sr0 is identical to the one set by OpenBIOS and SDR1 is unchanged so translations via the page table should still work, shouldn't it?)

I was mistaken about the values being the same as it is zeroing sr0. So can this explain why translation via the page table fails after this and why an ISI is generated? Why are the sr registers set up with the values above by OpenBIOS? Could they be 0 instead?

SR registers are used to translate EAs to VAs. If you set them all to 0 they would end up getting the same VSID.

OK but why is SEGR_BASE defined as 0x0400 in arch/ppc/qemu/ofmem.c?

I guess to make it easier to debug vs empty SR registers and to make sure it doesn't collide with a guest that sets SR registers to 0 temporarily?

What does real Apple firmware use for their VSIDs?

I'll modify oftest to print sr regs and try to rerun it to find out.

0x0041cdf8: isync

htab_base 000000000fe00000 htab_mask 000000000000ffff hash 000000000000041c
0 htab=000000000fe00000/000000000000ffff vsid=0 ptem=1 hash=000000000000041c

ppc_store_sdr1: 00000000

htab_base 0000000000000000 htab_mask 000000000000ffff hash 000000000000041c
0 htab=0000000000000000/000000000000ffff vsid=0 ptem=1 hash=000000000000041c
1 htab=0000000000000000/000000000000ffff vsid=0 api=1 hash=fffffffffffffbe3
Raise exception at 0041cdfc => 00000003 (40000000)
0x00000400:  mtsprg  2,r2

What comes after this is loading some values from the stack then disabling MMU bits in MSR (then putting the values loaded into BAT regs). If I could get these run without getting an ISI inbetween it would boot:

  41ce0c:       82 84 00 14     lwz     r20,20(r4)
  41ce10:       82 a4 00 10     lwz     r21,16(r4)
  41ce14:       82 c4 00 1c     lwz     r22,28(r4)
  41ce18:       82 e4 00 18     lwz     r23,24(r4)
  41ce1c:       83 05 00 04     lwz     r24,4(r5)
  41ce20:       83 25 00 00     lwz     r25,0(r5)
  41ce24:       83 45 00 0c     lwz     r26,12(r5)
  41ce28:       83 65 00 08     lwz     r27,8(r5)
  41ce2c:       83 85 00 14     lwz     r28,20(r5)
  41ce30:       83 a5 00 10     lwz     r29,16(r5)
  41ce34:       83 c5 00 1c     lwz     r30,28(r5)
  41ce38:       83 e5 00 18     lwz     r31,24(r5)
  41ce3c:       7c 00 04 ac     sync
  41ce40:       7c 00 00 a6     mfmsr   r0
  41ce44:       70 00 ff cf     andi.   r0,r0,65487
  41ce48:       7c 00 04 ac     sync
  41ce4c:       7c 00 01 24     mtmsr   r0
  41ce50:       4c 00 01 2c     isync

Why are we getting ISIs on QEMU and why can this work on real hardware without crashing?

We are getting ISIs because the HTAB is now 0 bytes long, so it doesn't contain any entries. Maybe it works on real hardware by accident because the translations are still in the TLB?

If so then why is not that the case on QEMU and can it be changed? Is the TLB flushed differently than real hardware? But I suspect the cache to be a more likely explanation.

I think with MorphOS you're best off detecting that you are running MorphOS somehow (checksum over loaded files, specific client interface calls only it does) and set MSR.DR=0 and MSR.IR=0.

This is extremely fragile and dependent on an exact version of executable that I like to avoid. Is there a problem with always disabling MMU bits when our vectors are overwritten? I guess that a well behaved guest should disable these before overwriting the vectors and enable them afterwards and for a buggy guest this would prevent random crashes or may make it work as in case of MorphOS. The problem is I don't know how to detect if the vectors are overwritten. I though about checking it from dsi_exception handler, but here I don't get more than one hits not like the ISI-s I don't want to happen. Do any of you have an idea about how could this be done?


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