xen: arm: inject unhandled instruction and data aborts to the guest.

xen: arm: inject unhandled instruction and data aborts to the guest.

Currently an unhandled data abort in guest context leads to us killing the
guest and an unhandled instruction abort in guest context leads to us killing
the host!

Andre pointed out that an unhandled data abort can be caused by e.g. dmidecode
looking for things which are not there in the guests physical address space.
Propagating the fault to the guest allows it to properly SIGSEGV the
processes.

A guest kernel can trivially jump to an unmapped physical address which would
cause an instruction abort. Killing the host for that is obviously bad.
Instead inject the exception so the guest kernel can SIGSEGV or panic() etc as
it deems appropriate.

Tested on arm64 (Mustang) and arm32 (Midway) with a dom0 kernel late_initcall
which either dereferences or jumps to address 0, provoking both behaviours and
resulting correctly in a guest kernel panic. Also tested on fast models with a
32-bit dom0 on a 64-bit hypervisor, which behaved correctly.

In addition tested on both platforms with a userspace program which either
calls to or dereferences address 0. The process is correctly killed with SEGV.

Lastly tested on Mustang with a 32-bit version of the userspace test on a
64-bit dom0 kernel.

I think that covers all the cases.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Cc: Andre Przywara <andre.przywara@calxeda.com>
[ ijc -- fixed up whitespace in if statements in cpsr_mode_switch ]