Translation regimes that only support one virtual address space (such as
the ones for EL2 and EL3) can flag memory regions as execute-never by
setting to 1 the XN bit in the Upper Attributes field in the translation
tables descriptors. Translation regimes that support two different
virtual address spaces (such as the one shared by EL1 and EL0) use bits
PXN and UXN instead.

The Trusted Firmware runs at EL3 and EL1, it has to handle translation
tables of both translation regimes, but the previous code handled both
regimes the same way, as if both had only 1 VA range.

When trying to set a descriptor as execute-never it would set the XN
bit correctly in EL3, but it would set the XN bit in EL1 as well. XN is
at the same bit position as UXN, which means that EL0 was being
prevented from executing code at this region, not EL1 as the code
intended. Therefore, the PXN bit was unset to 0 all the time. The result
is that, in AArch64 mode, read-only data sections of...