Comment 1 for bug 1892540

This happens because openbios accesses unassigned memory during the SBus scan:

Probing SBus slot 0 offset 0
invalid accepts: (null) addr 20000000 size: 1
Probing SBus slot 1 offset 0
invalid accepts: (null) addr 30000000 size: 1
Probing SBus slot 2 offset 0
invalid accepts: (null) addr 40000000 size: 1
Probing SBus slot 3 offset 0
Probing SBus slot 4 offset 0
invalid accepts: (null) addr 60000000 size: 1
Probing SBus slot 5 offset 0

Thread 4 "qemu-system-spa" hit Breakpoint 1, memory_region_access_valid (mr=0x555555df20c0 <io_mem_unassigned>,
    addr=536870912, size=1, is_write=<optimized out>, attrs=...)
    at .../softmmu/memory.c:1358
1358 return false;

(gdb) list

1355 if (mr->ops->valid.accepts
1356 && !mr->ops->valid.accepts(mr->opaque, addr, size, is_write, attrs)) {
1357 fprintf(stderr, "invalid accepts: %s addr %"PRIx64 " size: %d\n", mr->name, addr, size);
1358 return false;
1359 }

(gdb) p mr->ops->valid.accepts
$1 = (_Bool (*)(void *, hwaddr, unsigned int, _Bool, MemTxAttrs)) 0x555555736f10 <unassigned_mem_accepts>

(gdb) list unassigned_mem_accepts
1271
1272 static bool unassigned_mem_accepts(void *opaque, hwaddr addr,
1273 unsigned size, bool is_write,
1274 MemTxAttrs attrs)
1275 {
1276 return false;
1277 }