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When AP1 memory is being emulated (long descriptor page tables are in
use), the AbortTrap machinery is used to emulate usermode read access.
This provides coverage for all read instructions except those that
AbortTrap handles via MemMap requests - LDREX, LDA, LDAEX, LDF & LFM.

LDREX & LDAEX request both read & write access, so are fine (the MemMap
request will get passed through to the registered AbortTrap handlers).

LDF & LFM are irrelevant, since they only exist on ARM7500FE (on other
machines FPEmulator will translate them to regular LDR/LDM, which are
handled correctly)

LDA however, will generate a plain "memmap with usermode read" request.
When AbortTrap looks at the permissions of emulated AP1 it doesn't take
into account the fact that the usermode read permission is being
emulated, so it thinks that everything is fine and claims the memmap
was successful, causing the abort handler to retry the instruction
without making any changes, resulting in an infinite abort loop.

Deal with this by detecting the above situation and also requesting
usermode execute access. This will avoid the kernel (and hopefully the
registered AbortTrap handlers) from thinking that the emulated AP1 is
acceptable, without adversely affecting the behaviour of other
instructions or access privileges. If no handler is present or the
memmap request is denied, the abort will get passed on to the next stage
of the abort handler (i.e. you'll get a standard data abort from trying
to LDA from arbitrary emulated AP1 memory)

The new test program (Dev/AbortTrap/attest_ap1) will check that this
edge case is dealt with correctly.

Tested on Pi 4, for both long & short page tables

Version 6.59. Tagged as 'Kernel-6_59'

Sadly we need one file per combination of action files, but by adding
these pre-generated cache files to git we can speed up building the
kernel from clean by a significant amount.

This supports all the load/store instructions, including FPA & VFP/NEON.
Most instructions are handled directly via the base version of the
AbortTrap API that was first implemented in RISC OS Select. However, to
properly cope with LDREX/STREX, and future support for prefetch aborts,
the API has been extended to allow the kernel to request that a block of
memory is mapped in with certain permissions. For LDREX/STREX the kernel
will then rewind the PC so that the instruction can be retried directly.

Test code in Dev/AbortTrap exists in order to allow checking of all
major functionality, along with code for building the code in a
softloadable module for easier/quicker testing.