feat: add Zicfiss (shadow stack) CSR support

spec/std/isa/csr/ssp.yaml

@@ -0,0 +1,49 @@
+# Copyright (c) Katherine Hsu
+# SPDX-License-Identifier: BSD-3-Clause-Clear
+
+# yaml-language-server: $schema=../../../schemas/csr_schema.json
+
+$schema: "csr_schema.json#"
+kind: csr
+name: ssp
+long_name: Shadow Stack Pointer
+address: 0x011
+description:
+  - id: csr-ssp-purpose
+    normative: true
+    text: |
+      The `ssp` CSR is an unprivileged read-write (URW) CSR that reads and writes
+      XLEN low order bits of the shadow stack pointer. The `ssp` is always as wide
+      as the XLEN of the current privilege mode. The bits 1:0 of ssp are read-only
+      zero. If the UXLEN or SXLEN may never be 32, then the bit 2 is also read-only
+      zero.
+priv_mode: U
+length: XLEN
+definedBy: Zicfiss
+fields:
+  VALUE:
+    location_rv64: 63-3
+    location_rv32: 31-2
+    description: |
+      The value in ssp points to the top of the shadow stack, which is the address
+      of the last element stored on the shadow stack.
+    type: RW
+    reset_value: UNDEFINED_LEGAL
+    sw_write(csr_value): |
+      if ((mode() < PrivilegeMode::M && CSR[menvcfg].SSE == 0) ||
+          (mode() == PrivilegeMode::U && CSR[senvcfg].SSE == 0)) {
+          raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
+      } else if ((mode() == PrivilegeMode::VS && CSR[henvcfg].SSE == 0) ||
+                 (mode() == PrivilegeMode::VU && (CSR[henvcfg].SSE == 0 || CSR[senvcfg].SSE == 0)) {
+          raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
+      } else {
+        return csr_value.VALUE
+      }
+sw_read(): |
+  if ((mode() < PrivilegeMode::M && CSR[menvcfg].SSE == 0) ||
+      (mode() == PrivilegeMode::U && CSR[senvcfg].SSE == 0)) {
+      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
+  } else if ((mode() == PrivilegeMode::VS && CSR[henvcfg].SSE == 0) ||
+             (mode() == PrivilegeMode::VU && (CSR[henvcfg].SSE == 0 || CSR[senvcfg].SSE == 0)) {
+      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
+  }