Strange GDB behaviour when modifying MMIO

[mchack-work]

When attaching GDB to the running qemu, for instance started with -s and -S, I expect to be able to examine and write to the memory mapped to hardware functions, too. Maybe this is a misunderstanding that it's possible? Or did we forget something? Let's investigate.

For instance:

(gdb) set *0x40000000 = 0xabababab
(gdb) x/4b 0x40000000
0x40000000:	0xab	0xab	0xab	0xab
(gdb) set *0xd0000000 = 0xabababab
(gdb) x/4b 0xd0000000
0xd0000000:	0x00	0x00	0x00	0x00

What? Here I'm modifying RAM at 0x40000000. That works OK. Then I'm trying to change FW_RAM at 0xd0000000. Nope.

I added debug prints to tk1_mmio_read() and _write(). Only read() is called.