One can change the base register representation from an XLEN-length
bitvector by supplying a different definition of the regtype type,
and functions that convert between that type and the default
XLEN-length bitvector. The interface for this is specified in
riscv_reg_type.sail. An extension can implement a different
representation in a file that follows that interface, and use it with
the rest of the model.
Adding registers (such as for floating-point) would involve naming
them and defining their read and write accessors, as is done for the
integer registers in riscv_types.sail, riscv_reg_type.sail and
riscv_regs.sail. For modularity, these new definitions should be
added in separate files. If any of these registers are
control-and-status registers (CSRs), or depend on
privilege level (such as hypervisor-mode registers), additional access
control checks would need to be provided as is done for the standard
CSRs in riscv_sys_regs.sail and riscv_sys_control.sail. Access to
newly added CSRs can be hooked in riscv_csr_ext.sail. In addition,
the bits mstatus.XS and mstatus.SD may need to be updated
or extended to handle any extended register state.
Adding a new privilege level or functionality restricted by privilege
level will normally be accompanied by defining new exception causes
and their encodings. This will require modifying and extending the
existing definitions for privilege levels and exceptions in
riscv_types.sail, and modifying the exception handling and privilege
transition functions in riscv_sys_control.sail.
An extension that needs to interact closely with exception handling
may need to capture additional information at the time of an
exception. This is supported using the ext field in the
sync_exception type in riscv_sync_exception.sail, which is where
the extension can store this information. The addresses involved in
exception handling can be modified by following the interface provided
in riscv_sys_exceptions.sail. New exception codes can be introduced
using the E_Extension variant of the ExceptionType in
riscv_types.
Adding support for new devices such as interrupt controllers and
similar memory-mapped I/O (MMIO) entities strictly falls outside the
purview of the formal model itself, and typically is not done
directly in the Sail model. However, bindings to this external
functionality can be provided to Sail definitions using the extern
construct of the Sail language. riscv_platform.sail can be examined
to see how this is done for the SiFive core-local interrupt (CLINT)
controller, the HTIF timer and terminal devices. The
implementation of the actual functionality provided by these MMIO
devices would need to be added to the C and OCaml emulators.
If this functionality requires the definition of new interrupt
sources, their encodings would need to be added to riscv_types.sail,
and their delegation and handling added to riscv_sys_control.sail.
Physical memory addressing and access is defined in riscv_mem.sail.
Any new regions of memory that are accessible via physical addresses
will require modifying the mem_read, mem_write_value or their
supporting functions checked_mem_read and checked_mem_write.
The model supports storing and retrieving metadata along with memory
values at each physical memory address. The default interface for
this is defined in prelude_mem_metadata.sail. An extension can
customize the default implementation there to support its metadata
type.
The actual content of such memory, and its modification, can be
defined in separate Sail files. This functionality will have access
to any newly defined architectural state. One can examine how normal
physical memory access is implemented in riscv_mem.sail with helpers
in prelude_mem.sail and prelude_mem_metadata.sail.
Virtual memory is implemented in riscv_vmem.sail, and defining new
address translation schemes will require modifying the top-level
translateAddr function. New types of memory access can be defined
using the definitions in riscv_vmem_types. Any access control
checks on virtual addresses and the specifics of the new address
translation can be specified in a separate file. This functionality
can access any newly defined architectural state.
The RV64 architecture has reserved bits in the PTE that can be
utilized for research experimentation. These bits can be accessed and
modified using the ext_pte argument in functions implementing the
page-table walk. The information computed by and used during the
page-table can also be varied using the ext_ptw argument, which can
be defined and used by extensions as needed. Extensions can override
the definitions of checkPTEPermission and ext_get_ptw_error to
generate and process this custom information, and
ext_translate_exception to convert any custom errors into
extension-specific exceptions.
An extension may wish to perform validity checks and transformations
on addresses generated by an instruction before a memory access is
performed with the generated address. This is supported using the
types defined in riscv_addr_checks_common.sail, with a default
implementation in riscv_addr_checks.sail.
The handling of the memory addresses involved during exception
handling can be extending using the interface defined in
riscv_sys_exceptions.sail.
An extension might want to similarly check and transform accesses to
the program counter. This is supported by supplying implementations
of the functions defined in riscv_pc_access.sail.
In addition, dynamically enabling and disabling the RVC extension has
an effect on legal PC alignment; in particular, attempts to disable
RVC are ignored if the PC becomes unaligned in the base architecture.
Extensions can also veto these attempts by appropriately defining
ext_veto_disable_C.
This is typically simpler than adding new architectural state or
memory interposition. Each new set of instructions can be specified
in a separate self-contained file, with their instruction encodings,
assembly language specifications and the corresponding encoders and
decoders, and execution semantics. riscv.sail can be examined for
examples on how this can done. These instruction definitions can
access any newly defined architectural state and perform virtual or
physical memory accesses as is done in riscv.sail.
An extension may wish to check and transform a decoded instruction.
This is supported via a post-decode extension hook, the default
implementation of which is provided in riscv_decode_ext.sail.
For any new extension, it is helpful to factor it out into the above items. When specifying and implementing the extension, it is expected to be easier to implement it in the above listed order.
As an example, one can examine the implementation of the 'N' extension
for user-level interrupt handling. The architectural state to support
'N' is specified in riscv_next_regs.sail, added CSR and control
functionality is in riscv_next_control.sail, and added instructions
are in riscv_insts_next.sail. Access to the CSRs added by the
extension are hooked in riscv_csr_ext.sail.
In addition, privilege transition and interrupt delegation logic in
riscv_sys_control.sail has been extended.