Commit 56aebc89 changed gdbstub in way
that debugging 32 or 16-bit guest code is no longer possible with qemu
for x86_64 guest CPUs. Since that commit, qemu only provides registers
sets for 64-bit, forcing current and foreseeable gdb to also switch its
architecture to 64-bit. And this breaks if the inferior is 32 or 16 bit.

No question, this is a gdb issue. But, as it was confirmed in several
discusssions with gdb people, it is a non-trivial thing to fix. So until
qemu finds a gdb version attach with a rework x86 support, we have to
work around it by switching the register layout as the guest switches
its execution mode between 16/32 and 64 bit.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>