Power

Also a few programs that run well on AIX6.1 on AIX7.2 got segmentation fault, why?

# wzappkey -p asftp
Segmentation fault(coredump)

# ps
Segmentation fault(coredump)

Your question is very confusing.
On what are you running QEMU - HW and OS?

Are you emulating the POWER HW too?

If you are running AIX 6.1 binaries on native AIX 7.2 (no need for QEMU)  the some programs are HW dependant and use grubby techniques to get the data - for example ps deals with /dev/kmem to get data from the processes that are not available in the kernel.  Other problem commands are dbx, some low level performance tools have to read adapter memory for extra stats and anything that reads data from the firmware which has changed down through the years. But why do this at all - run the AIX7.2 version of these commands

If you are running QEMU on AIX 7.2  and then running AIX 6.1 in QEMU - Again what is the point?  Run the AIX 6.1 applications native directly on AIX 7.2 - AIX has excellent backward compatibility. So system admin commands use AIX 7.2 and application code run the AIX 6.1 version.  Heck, AIX 5.2 and AIX 5.3 applications will probably run too.   Under QEMU you are likely to have issues as the QEMU id emulating firmware access which may not be perfect.  This is a QEMU issue and not an AIX issue. You might make better progress if QEMU is emulating older POWER Chips

Hope this helps, N