libsel4arm-vmm: add support for decoding and emulating arm instructions

[kent-mcleod]

(Moved from: https://sel4.atlassian.net/browse/SELFOUR-793) Currently libsel4arm-vmm provides basic support for decoding and emulating faults for the TK1. This works by relying on the HSR register providing a valid instruction syndrome.

From the armv7 manual:

"A valid instruction syndrome provides information that can help a hypervisor to emulate the

instruction efficiently. Instruction syndromes are returned for instructions for which such

accelerated emulation is possible."

The syndrome contains what register is used for the read/write, what the width of the data is, whether it needs to be sign extended. This makes the fault easy to decode and emulate which libsel4arm-vmm currently does.

However a valid syndrome only occurs for a subset of arm data instructions. For instructions outside of this the instruction needs to be decoded and properly emulated (such as handling register write back where the second register that holds the address of the lookup is updated with the immediate offset such as pop and push instructions). Currently this is not supported.

This only becomes a problem when the vmm wants to trap and emulate faults. Memory protection faults that result in needing to map a page into the mmu doesn't require decoding any instructions because the fault address is stored in a different register and the fault is normally handled by mapping in a page and then restarting the faulting instruction rather than advancing the fault instruction which requires the instruction to be emulated.

We currently use trap and emulation for providing more granular memory access control than the frame level. Up until now this lack of support hasn't been an issue but could become an issue in the future.