FEX emulates faulting instructions (e.g. ud2 or int 2d) by jumping to
the dispatcher and filling out a structure with fault details in the
thread context. Parse this out into a windows exception record structure
so the correct fault information can be seen by the guest.
As the exception dispatcher is initially invoked on the emulator stack,
control needs to be transferred to the dispatcher on the guest stack
after recovering the x86 RSP to allow for invoking x86 exception
handlers.
This is used by the kernel (or UNIX side of ntdll in wine) to jump into
x86 code with the given context as is necessary when e.g. returning from
an exception.
FEX is unable to deal with reentrant compilation of any x64 hotpatches
so they need to be ignored by bypassing FFSs and calling directly into
the native target.
ARM64 requires that SP is always 16-byte aligned for memory accesses,
but ARM64EC shares the SP between x64 code and ARM64 code, the former
of which doesn't enforce such a restriction. This causes crashes in
programs such as HITMAN 3 that don't correctly follow the Windows ABI
and call into system library functions with SP only 8-byte-aligned.
Fixup stack alignment in such cases by leaving the 8-byte return
address on the stack and returning to a lone 'ret' instruction instead.
This allows for running x64 applications under wine without having to run all
of wine under FEX. The JIT is invoked when ARM64EC code performs an indirect
branch to x64 code, and left whenever the x64 code calls into ARM64EC
code.
Billy Laws