ARM: 9159/1: decompressor: Avoid UNPREDICTABLE NOP encoding

ARM: 9159/1: decompressor: Avoid UNPREDICTABLE NOP encoding

In the decompressor's head.S we need to start with an instruction that
is some kind of NOP, but also mimics as the PE/COFF header, when the
kernel is linked as an UEFI application. The clever solution here is
"tstne r0, #0x4d000", which in the worst case just clobbers the
condition flags, and bears the magic "MZ" signature in the lowest 16 bits.

However the encoding used (0x13105a4d) is actually not valid, since bits
[15:12] are supposed to be 0 (written as "(0)" in the ARM ARM).
Violating this is UNPREDICTABLE, and *can* trigger an UNDEFINED
exception. Common Cortex cores seem to ignore those bits, but QEMU
chooses to trap, so the code goes fishing because of a missing exception
handler at this point. We are just saved by the fact that commonly (with
-kernel or when running from U-Boot) the "Z" bit is set, so the
instruction is never executed. See [0] for more details.

To make things more robust and avoid UNPREDICTABLE behaviour in the
kernel code, lets replace this with a "two-instruction NOP":
The first instruction is an exclusive OR, the effect of which the second
instruction reverts. This does not leave any trace, neither in a
register nor in the condition flags. Also it's a perfectly valid
encoding. Kudos to Peter Maydell for coming up with this gem.

[0] https://lore.kernel.org/qemu-devel/YTPIdbUCmwagL5%2FD@os.inf.tu-dresden.de/T/

Link: https://lore.kernel.org/linux-arm-kernel/20210908162617.104962-1-andre.przywara@arm.com/T/
Fixes: 81a0bc39ea19 ("ARM: add UEFI stub support")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Adam Lackorzynski <adam@l4re.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>