target/arm: Make WFI a NOP for userspace emulators

target/arm: Make WFI a NOP for userspace emulators

The WFI insn is not system-mode only, though it doesn't usually make
a huge amount of sense for userspace code to execute it.  Currently
if you try it in qemu-arm then the helper function will raise an
EXCP_HLT exception, which is not covered by the switch in cpu_loop()
and results in an abort:

qemu: unhandled CPU exception 0x10001 - aborting
R00=00000001 R01=408003e4 R02=408003ec R03=000102ec
R04=00010a28 R05=00010158 R06=00087460 R07=00010158
R08=00000000 R09=00000000 R10=00085b7c R11=408002a4
R12=408002b8 R13=408002a0 R14=0001057c R15=000102f8
PSR=60000010 -ZC- A usr32
qemu:handle_cpu_signal received signal outside vCPU context @ pc=0x7fcbfa4f0a12

Make the WFI helper function return immediately in the usermode
emulator. This turns WFI into a NOP, which is OK because:
 * architecturally "WFI is a NOP" is a permitted implementation
 * aarch64 Linux kernels use the SCTLR_EL1.nTWI bit to trap
   userspace WFI and NOP it (though aarch32 kernels currently
   just let WFI do whatever it would do)

We could in theory make the translate.c code special case user-mode
emulation and NOP the insn entirely rather than making the helper
do nothing, but because no real world code will be trying to
execute WFI we don't care about efficiency and the helper provides
a single place where we can make the change rather than having
to touch multiple places in translate.c and translate-a64.c.

Fixes: https://bugs.launchpad.net/qemu/+bug/1926759
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210430162212.825-1-peter.maydell@linaro.org