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Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 fa

From: BALATON Zoltan
Subject: Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 family
Date: Fri, 26 Nov 2021 15:46:50 +0100 (CET)

On Fri, 26 Nov 2021, Fabiano Rosas wrote:
BALATON Zoltan <balaton@eik.bme.hu> writes:

On Wed, 24 Nov 2021, Fabiano Rosas wrote:
Fabiano Rosas <farosas@linux.ibm.com> writes:

Hi all,

We have this bug in QEMU which indicates that we haven't been able to
run openbios on a 7450 cpu for quite a long time:


  $ ./qemu-system-ppc -serial mon:stdio -nographic -cpu 7410

>> =============================================================
>> OpenBIOS 1.1 [Nov 1 2021 20:36]

  $ ./qemu-system-ppc -serial mon:stdio -nographic -cpu 7450 -d int

This CPU appears in PowerMac G4 so maybe better use -machine mac99,via=pmu
with it as it's strange to put it in a g3beige but that may not matter for
reproducing the problem.

As for guests, those running on the said PowerMac G4 should have support
for these CPUs so maybe you can try some Mac OS X versions (or maybe
MorphOS but that is not the best for debugging as there's no source
available nor any help from its owners but just to see if it boots it may
be sufficient, it should work on real PowerMac G4). According to
<https://en.wikipedia.org/wiki/PowerPC_G4#PowerPC_7450> this CPU was used
in <https://en.wikipedia.org/wiki/Power_Mac_G4#Digital_Audio/Quicksilver>
and it runs up to Mac OS 10.4.11. (Although OpenBIOS sets the device tree
according to a PowerMac3,1 so not sure it's entirely correct for the
PowerMac3,5 that has a 7450 CPU and if it matters for Mac OS X.)

I asked about this before but got no reply back then:

This was because pegasos2 should have 7447 but it did not work so
currently I've set it to 7400 which also works. The original board
firmware had some problem detecting it but I think that only results in
wrong CPU speed shown which is only a cosmetic problem, otherwise it seems
to work. Since pegasos2 does not use OpenBIOS but either VOF or the
board's original firmware it may be an alternative way to test at least
7447 which the firmware and guests running on that board should work with.
At least Debian 8.11 powerpc version had support for pegasos2 and should
boot, I'm not sure newer versions still work. More info on pegasos2 can be
found at:

I don't remember what problem I had with 7447 but if it does not work with
pegasos2 then maybe there's some other problem with it too. I think it was
maybe related to TLBs but I don't know and had no time to try again so I
could be entirely wrong about this.

So yesterday I tested these:

* all with my patched OpenBIOS
** all of them work fine with the 7400 CPU

- macos9 w/ -M mac99 and -cpu 7450

OS starts and then freezes. Upon further inspection I see that it has
the 0x1000, 0x1100 and 0x1200 vectors implemented, but not the 7450
ones. It implements the 6xx SW TLB handler, i.e. it accesses SPR 976
instead of 980.

- macosx10 w/ -M mac99 and -cpu 7450

Shows the apple logo and then spins. Looking at the asm I don't see
anything resembling the 7450 software TLB code. I'm calling it unsupported.

- debian 10 w/ -M mac99 and -cpu 7450

Bevare that -M mac99 is not matching the device tree as it has ADB instead of USB but claims to be PowerMac3,1 nevertheless. This is a silly default preserved for compatibility with some older OS X versions but to avoid problems it's better to always use -M mac99,via=pmu which is the closest to a real PowerMac3,1. There is some info on this and which option to use with which version at https://www.emaculation.com/doku.php/ppc-osx-on-qemu-for-osx

Boots linux and then spins. It has the vectors implemented, but it also
uses different SPRs. The data misses come via 976, which is different
from 7450, which uses only 980 (tlbmiss) for instruction and data.

- morphos w/ -M pegasos2 and -cpu 7447|7450

Hangs. It also has a different software TLB model implemented:
Trying to read invalid spr 978 (0x3d2) at 00001100
Trying to read invalid spr 977 (0x3d1) at 0000110c
Trying to read invalid spr 979 (0x3d3) at 0000115c
Trying to read invalid spr 976 (0x3d0) at 00001188

So my initial impression that no OS supports the 7450 software TLB seems
to match these findings and what people have said elsewhere in the

I'm getting this with MorphOS on pegasos2:

$ qemu-system-ppc -M pegasos2 -device ati-vga,romfile="" -kernel boot.img -cdrom morphos-3.15.iso -serial stdio -d unimp,guest_errors,int 2>&1 | grep --line-buffered -v '^Invalid read\|^hypercall\|^syscall\|^Raise exception.*0000000[48a]'

Memory used before SYS_Init: 9MB
i8259: level sensitive irq not supported
i8259: level sensitive irq not supported

unsupported keyboard cmd=0xaf
PCI ATA/ATAPI Driver@2: PIO Mode 4
PCI ATA/ATAPI Driver@2: UDMA Mode 5
ide.device@2: QEMU     QEMU DVD-ROM     <CDROM>

and it boots with the default 7400 CPU but with -cpu 7447 I get:

Memory used before SYS_Init: 9MB
i8259: level sensitive irq not supported
i8259: level sensitive irq not supported
Raise exception at 00051500 => 0000004f (00)
Trying to read invalid spr 978 (0x3d2) at 00001100
Trying to read invalid spr 977 (0x3d1) at 0000110c
Trying to read invalid spr 979 (0x3d3) at 0000115c
Trying to read invalid spr 976 (0x3d0) at 00001188

ExceptionThread: Exception Thread 0xbc8030 <System Init>
ExceptionThread: Type 0x3 <Data Access>
ExceptionThread: Illegal Data Access at 0x40000000 DSISR 0x40000000 <Read Access 
to a not existing Page>
PC(SRR0) 0x00051500 MSR(SRR1) 0x0000b030
 CR 00000884 XER 20000000 LR 0002166c CTR 0001a880
General Register Dump:
GPR[0] 00021000 40000000 0000b030 0200b030 008a1c9c 1fc59000 00000000 00000000
GPR[8] 00be8718 00000000 00000000 00053e3c 0200b030 0d0d0d0d 0e0e0e0e 0f0f0f0f
GPR[16] 10101010 11111111 12121212 13131313 14141414 15151515 16161616 00000002
GPR[24] 0081c1d0 00000000 10020010 00000000 10000013 00000004 00030000 00000001
FPSCR 00000000
FPECR 00000000
FPR[0] 0x0000000020000002 0x0000000000000000 0x0000000000000000 
FPR[4] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[8] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[12] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[16] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[20] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[24] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
FPR[28] 0x0000000000000000 0x0000000000000000 0x0000000000000000 
Special Register Dump:
  IABR 00000000  DABR 00000000
  PCM1 00000000  PMC2 00000000 PMC3 00000000 PMC4 00000000
OS Register Dump:
 SysCall_SRR0 00053e3c SysCall_SRR1 0200b030 SysCall_GPR2 008a17b0 SysCall_LR 
0d0d0d0d SysCall_CR 00052624
 SRR0[] 00000000 00000000 00000000 00000000

StackFrame History:
Stack 0x40000000 StackEnd 0xbc7030 LR 0x2166c

Exception 4f seems to be POWERPC_EXCP_DLTLB which does not seem to happen with 7400 CPU, neither the SPR reads so there is some difference with 7447 but not sure what do these mean, I don't know much about these CPUs and did not read the docs. There is some debug code that can be enabled with DEBUG_SOFTWARE_TLB in target/ppc/excp_helper.c to get more info but I haven't tried that, looks like MorphOS is not expecting these so normally would not use it.

The pegasos2 firmware seems to work but detects the CPU differently. With 7400:

cpu0: PowerPC,G4 CPUClock 599 Mhz BUSClock 133 Mhz (Version 0x000C,0x0209)

The 600 Mhz CPU clock is wrong but since the only two CPUs available were a 600 MHz G3 or a 1GHz 7447 it probably thinks anything not the 7447 or newer is a G3 that was running at 600 MHz. But with the 7447 it's also wrong:

cpu0: PowerPC,74x7 CPUClock 1533 Mhz BUSClock 133 Mhz (Version 0x8002,0x0101)

which should be 1 GHz (or 999 MHz), maybe the clock multiplier is not set correctly but not sure where it gets that from. Otherwise the firmware seems to work with 7447 CPU as well and get the same exception as above when tring boot MorphOS.


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