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DOUBLE MMU FAULT when running -M virt in qemu-system-m68k
Description of problem:
When running qemu-system-m68k with the -M virt machine type, a DOUBLE MMU FAULT occurs immediately upon startup, even without any BIOS, disk image, or additional configuration.
Steps to reproduce:
1. qemu-system-m68k -M virt -m 4M -serial stdio
QEMU crashes immediately with the following output:
```
qemu: fatal: DOUBLE MMU FAULT
D0 = 00000000 A0 = 00000000 F0 = 7fff ffffffffffffffff ( nan)
D1 = 00000000 A1 = 00000000 F1 = 7fff ffffffffffffffff ( nan)
D2 = 00000000 A2 = 00000000 F2 = 7fff ffffffffffffffff ( nan)
D3 = 00000000 A3 = 00000000 F3 = 7fff ffffffffffffffff ( nan)
D4 = 00000000 A4 = 00000000 F4 = 7fff ffffffffffffffff ( nan)
D5 = 00000000 A5 = 00000000 F5 = 7fff ffffffffffffffff ( nan)
D6 = 00000000 A6 = 00000000 F6 = 7fff ffffffffffffffff ( nan)
D7 = 00000000 A7 = 00000000 F7 = 7fff ffffffffffffffff ( nan)
PC = 00400000 SR = 2704 T:0 I:7 SI --Z--
FPSR = 00000000 ----
FPCR = 0000 X RN
A7(MSP) = 00000000 A7(USP) = 00000000 ->A7(ISP) = 00000000
VBR = 0x00000000
SFC = 0 DFC 0
SSW 00000105 TCR 00000000 URP 00000000 SRP 00000000
DTTR0/1: 00000000/00000000 ITTR0/1: 00000000/00000000
MMUSR 00000000, fault at fffffffc
```
Additional information:
The issue seems to be related to incorrect memory initialization, causing a fault at address fffffffc.
The PC = 00400000 suggests that QEMU is jumping to an invalid address early in the boot process.
The fact that the fault is consistent across different configurations (q800, next-cube, etc) points to a possible regression or incomplete memory initialization in the virt machine.
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