Add JVT CSR (#782)

This PR closes Issue #562 and adds the missing Zcmt CSR (jvt.yaml).

---------

Signed-off-by: Katherine Hsu <[email protected]>
Co-authored-by: Derek Hower <[email protected]>

Author: neverlandiz

spec/std/isa/csr/jvt.yaml

@@ -0,0 +1,120 @@
+# 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: jvt
+long_name: Table Jump Base Vector and Control Register
+address: 0x017
+description:
+  - id: csr-jvt-purpose
+    normative: true
+    text: |
+      The `jvt` register is an XLEN-bit WARL read/write register that holds the jump table configuration,
+      consisting of the jump table base address (BASE) and the jump table mode (MODE).
+  - id: csr-jvt-architectural
+    normative: false
+    text: |
+      `jvt` CSR adds architectural state to the system software context (such as an OS process), therefore
+      must be saved/restored on context switches.
+priv_mode: U
+length: XLEN
+definedBy: Zcmt
+fields:
+  BASE:
+    location_rv64: 63-6
+    location_rv32: 31-6
+    description: |
+      The value in the BASE field must always be aligned on a 64-byte boundary. Note that the CSR contains only
+      bits XLEN-1 through 6 of the address base. When computing jump-table accesses, the lower six bits of base
+      are filled with zeroes to obtain an XLEN-bit jump-table base address `jvt.base` that is always aligned on a
+      64-byte boundary.
+
+      `jvt.base` is a virtual address, whenever virtual memory is enabled.
+
+      The memory pointed to by `jvt.base` is treated as instruction memory for the purpose of executing table
+      jump instructions, implying execute access permission.
+    type(): |
+      if (JVT_READ_ONLY) {
+        return CsrFieldType::RO;
+      }
+      else {
+        return CsrFieldType::RW;
+      }
+    reset_value: UNDEFINED_LEGAL
+    sw_write(csr_value): |
+      if ((mode() != PrivilegeMode::M) && implemented?(ExtensionName::Smstateen)) {
+        if (CSR[mstateen0].JVT == 1'b0) {
+          unimplemented_csr($encoding);
+        }
+      }
+      else if ((mode() == PrivilegeMode::U) && implemented?(ExtensionName::Ssstateen)) {
+        if (CSR[sstateen0].JVT == 1'b0) {
+          unimplemented_csr($encoding);
+        }
+      }
+      else if ((mode() == PrivilegeMode::VS) && implemented?(ExtensionName::Ssstateen)) {
+        if (CSR[hstateen0].JVT == 1'b0) {
+          unimplemented_csr($encoding);
+        }
+      }
+      else if ((mode() == PrivilegeMode::VU) && implemented?(ExtensionName::Ssstateen)) {
+        if ((CSR[sstateen0].JVT == 1'b0) || (CSR[hstateen0].JVT == 1'b0)) {
+          unimplemented_csr($encoding);
+        }
+      }
+      else {
+        if (JVT_BASE_TYPE == "custom") {
+          unpredictable("jvt.BASE has custom behavior");
+        } else {
+          return csr_value.BASE & (JVT_BASE_MASK >> 6);
+        }
+        return csr_value.BASE == 0 ? 0 : UNDEFINED_LEGAL_DETERMINISTIC;
+      }
+  MODE:
+    location: 5-0
+    description: |
+      `jvt.mode` is a WARL field, so can only be programmed to modes which are implemented. Therefore the
+      discovery mechanism is to attempt to program different modes and read back the values to see which
+      are available. Jump table mode must be implemented.
+    type(): |
+      if (JVT_READ_ONLY) {
+        return CsrFieldType::RO;
+      }
+      else {
+        return CsrFieldType::RW;
+      }
+    reset_value: 0
+    sw_write(csr_value): |
+      if (JVT_READ_ONLY || (csr_value.MODE != 0)) {
+        raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
+      }
+      return 0;
+sw_read(): |
+  # If the Smstateen extension is implemented, then bit 2 in `mstateen0`, `sstateen0`, and `hstateen0` is
+  # implemented. If bit 2 of a controlling `stateen0` CSR is zero, then access to the `jvt` CSR and execution
+  # of a `cm.jalt` or `cm.jt` instruction by a lower privilege level results in an illegal-instruction trap (or, if
+  # appropriate, a virtual-instruction trap).
+
+  if ((mode() != PrivilegeMode::M) && implemented?(ExtensionName::Smstateen)) {
+    if (CSR[mstateen0].JVT == 1'b0) {
+      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
+    }
+  }
+  else if ((mode() == PrivilegeMode::U) && implemented?(ExtensionName::Ssstateen)) {
+    if (CSR[sstateen0].JVT == 1'b0) {
+      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
+    }
+  }
+  else if ((mode() == PrivilegeMode::VS) && implemented?(ExtensionName::Ssstateen)) {
+    if (CSR[hstateen0].JVT == 1'b0) {
+      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
+    }
+  }
+  else if ((mode() == PrivilegeMode::VU) && implemented?(ExtensionName::Ssstateen)) {
+    if ((CSR[sstateen0].JVT == 1'b0) || (CSR[hstateen0].JVT == 1'b0)) {
+      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
+    }
+  }

spec/std/isa/ext/Zcmt.yaml

@@ -1,4 +1,4 @@
-# Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
+# Copyright (c) Qualcomm Technologies, Inc. and Katherine Hsu
 # SPDX-License-Identifier: BSD-3-Clause-Clear
 
 # yaml-language-server: $schema=../../schemas/ext_schema.json
@@ -95,6 +95,30 @@ versions:
       #   - [Zcmt, "1.0.0"]
 
 params:
+  JVT_BASE_TYPE:
+    schema:
+      type: string
+      enum: [mask, custom]
+    description: |
+      Type of the jvt.BASE CSR field. One of:
+
+        * mask: jvt.BASE contains one or more implemented bits, which are indicated by JVT_BASE_MASK.
+        * custom: Custom behavior. Will cause hart to enter 'unpredictable' state on a write to jvt.BASE.
+  JVT_BASE_MASK:
+    schema:
+      type: integer
+      minimum: 64
+      maximum: 0x7ffffffffffffff # 58 bits
+      required_if(): return JVT_BASE_TYPE == "mask";
+    description: |
+      Mask representing the implemented bits of jvt.BASE. Includes the implicitly-zero bits of jvt.BASE, so JVT_BASE_MASK[5:0] must always be 0.
+    extra_validation: |
+      assert JVT_BASE_MASK < 0xffffffc0 if MXLEN == 32
+  JVT_READ_ONLY:
+    schema:
+      type: boolean
+    description: |
+      If Zcmt is implemented, JVT is implemented, but can contain a read-only value
   MSTATEEN_JVT_TYPE:
     when:
       name: Smstateen