arch-riscv: Implement MHARTID CSR

This patch implements the MHARTID CSR by intercepting attempts to access
it, similar to the way accesses to the performance counters are
intercepted, to return the thread's context ID.

Change-Id: Ie14a31036fbe0e49fb3347ac0c3c508d9427a10d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16988
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

riscv: Get rid of some ISA specific register types.

Change-Id: Ie812cf1d42536094273ba2ec731c16cca38db100
Reviewed-on: https://gem5-review.googlesource.com/c/14466
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>

arch: cpu: Stop passing around misc registers by reference.

These values are all basic integers (specifically uint64_t now), and
so passing them by const & is actually less efficient since there's a
extra level of indirection and an extra value, and the same sized value
(a 64 bit pointer vs. a 64 bit int) is being passed around.

Change-Id: Ie9956b8dc4c225068ab1afaba233ec2b42b76da3
Reviewed-on: https://gem5-review.googlesource.com/c/13626
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

arch-riscv: Add interrupt handling

Implement the Interrupts SimObject for RISC-V. This basically just
handles setting and getting the values of the interrupt-pending and
interrupt-enable CSRs according to the privileged ISA reference chapter
3.1.14. Note that it does NOT implement the PLIC as defined in chapter
7, as that is used for handling external interrupts which are defined
based on peripherals that are available.

Change-Id: Ia1321430f870ff5a3950217266fde0511332485b
Reviewed-on: https://gem5-review.googlesource.com/c/14377
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

arch-riscv: Update CSR implementations

This patch updates the CSRs to match the RISC-V privileged specification
version 1.10. As interrupts, faults, and privilege levels are not yet
supported, there are no meaninful side effects that are implemented.
Performance counters are also not yet implemented, as they do not have
specifications. Currently they act as cycle counters.

Note that this implementation trusts software to use the registers
properly. Access protection, readability, and writeability of registers
based on privilege will come in a future patch.

Change-Id: I1de89bdbe369b5027911b2e6bc0425d3acaa708a
Reviewed-on: https://gem5-review.googlesource.com/7441
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>

arch-riscv: Restructure ISA description

This patch restructures the RISC-V ISA description to use fewer classes
and improve its ability to be extended with nonstandard extensions in
the future. It also cleans up the disassembly for some of the CSR and
system instructions by removing source and destination registers for
instructions that don't have any.

[Fix class UImmOp to have an "imm" member rather than "uimm".]
[Update disassembly generation for new RegId class.]

Change-Id: Iec1c782020126e5e8e73460b84e31c7b5a5971d9
Reviewed-on: https://gem5-review.googlesource.com/3800
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

riscv: add remote gdb support

This patch adds support for debugging with remote GDB to RISC-V. Using
GDB compiled with the RISC-V GNU toolchain, it is possible to pause
and continue execution, view debugging information, etc. As with the
rest of RISC-V, this does not support full-system mode.

Change-Id: I2d3a8be614725e1be4b4c283f9fb678a0a30578d
Reviewed-on: https://gem5-review.googlesource.com/2304
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

arch: [Patch 1/5] Added RISC-V base instruction set RV64I

First of five patches adding RISC-V to GEM5. This patch introduces the
base 64-bit ISA (RV64I) in src/arch/riscv for use with syscall emulation.
The multiply, floating point, and atomic memory instructions will be added
in additional patches, as well as support for more detailed CPU models.
The loader is also modified to be able to parse RISC-V ELF files, and a
"Hello world\!" example for RISC-V is added to test-progs.

Patch 2 will implement the multiply extension, RV64M; patch 3 will implement
the floating point (single- and double-precision) extensions, RV64FD;
patch 4 will implement the atomic memory instructions, RV64A, and patch 5
will add support for timing, minor, and detailed CPU models that is missing
from the first four patches (such as handling locked memory).

[Removed several unused parameters and imports from RiscvInterrupts.py,
RiscvISA.py, and RiscvSystem.py.]
[Fixed copyright information in RISC-V files copied from elsewhere that had
ARM licenses attached.]
[Reorganized instruction definitions in decoder.isa so that they are sorted
by opcode in preparation for the addition of ISA extensions M, A, F, D.]
[Fixed formatting of several files, removed some variables and
instructions that were missed when moving them to other patches, fixed
RISC-V Foundation copyright attribution, and fixed history of files
copied from other architectures using hg copy.]
[Fixed indentation of switch cases in isa.cc.]
[Reorganized syscall descriptions in linux/process.cc to remove large
number of repeated unimplemented system calls and added implmementations
to functions that have received them since it process.cc was first
created.]
[Fixed spacing for some copyright attributions.]
[Replaced the rest of the file copies using hg copy.]
[Fixed style check errors and corrected unaligned memory accesses.]
[Fix some minor formatting mistakes.]
Signed-off by: Alec Roelke

Signed-off by: Jason Lowe-Power <jason@lowepower.com>