target/arm: Allow Aarch32 exception return to switch from Mon->Hyp

target/arm: Allow Aarch32 exception return to switch from Mon->Hyp

In U-boot, we switch from S-SVC -> Mon -> Hyp mode when we want to
enter Hyp mode. The change into Hyp mode is done by doing an
exception return from Mon. This doesn't work with current QEMU.

The problem is that in bad_mode_switch() we refuse to allow
the change of mode.

Note that bad_mode_switch() is used to do validation for two situations:

 (1) changes to mode by instructions writing to CPSR.M
     (ie not exception take/return) -- this corresponds to the
     Armv8 Arm ARM pseudocode Arch32.WriteModeByInstr
 (2) changes to mode by exception return

Attempting to enter or leave Hyp mode via case (1) is forbidden in
v8 and UNPREDICTABLE in v7, and QEMU is correct to disallow it
there. However, we're already doing that check at the top of the
bad_mode_switch() function, so if that passes then we should allow
the case (2) exception return mode changes to switch into Hyp mode.

We want to test whether we're trying to return to the nonexistent
"secure Hyp" mode, so we need to look at arm_is_secure_below_el3()
rather than arm_is_secure(), since the latter is always true if
we're in Mon (EL3).

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190109152430.32359-1-agraf@suse.de
[PMM: rewrote commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>