feat: add support for Svrsw60t59b extension (#1135)

Author: jordancarlin

spec/std/isa/ext/Svrsw60t59b.yaml

@@ -0,0 +1,38 @@
+# Copyright (c) Jordan Carlin, Harvey Mudd College.
+# SPDX-License-Identifier: BSD-3-Clause-Clear
+
+# yaml-language-server: $schema=../../../schemas/ext_schema.json
+
+$schema: "ext_schema.json#"
+kind: extension
+name: Svrsw60t59b
+long_name: PTE Reserved-for-Software Bits 60-59
+type: privileged
+description: |
+  == "Svrsw60t59b" Extension for PTE Reserved-for-Software Bits 60-59, Version 1.0
+
+  If the Svrsw60t59b extension is implemented, then bits 60-59 of the page table
+  entries (PTEs) are reserved for use by supervisor software and are ignored by
+  the implementation.
+
+  If the Hypervisor (H) extension is also implemented, then bits 60-59 of the
+  G-stage PTEs are reserved for use by supervisor software and are ignored by the
+  implementation.
+
+  The Svrsw60t59b extension depends on Sv39.
+
+  [NOTE]
+  ====
+  Operating systems frequently use reserved bits within PTEs to store metadata for
+  advanced memory management features. Embedding these metadata bits directly within
+  the PTEs allows for fast access with minimal overhead, avoiding costly lookups in
+  auxiliary data structures. By default, Sv39 and Sv39x4 require a page fault and
+  a guest-page fault exception, respectively, to be raised if bits 60-59 are not
+  zero.
+  ====
+versions:
+  - version: "1.0.0"
+    state: ratified
+    ratification_date: 2025-08
+    requires:
+      name: Sv39

spec/std/isa/isa/globals.isa

@@ -1843,10 +1843,16 @@ function gstage_page_walk {
       PteFlags pte_flags = pte[9:0];
 
       # check if any reserved bits are set
-      # Sv32 has no reserved bits, and Sv39/48/57 all have reserved bits at 60:54
-      if ((VA_SIZE != 32) && (pte[60:54] != 0)) {
+      # Sv32 has no reserved bits, and Sv39/48/57 all have reserved bits at 58:54
+      if ((VA_SIZE != 32) && (pte[58:54] != 0)) {
         raise_guest_page_fault(op, gpaddr, vaddr, tinst, effective_mode);
       }
+      if (!implemented?(ExtensionName::Svrsw60t59b)) {
+        if ((PTESIZE >= 64) && pte[60:59] != 0) {
+          # 60:59 are reserved if Svrsw60t59b is not supported
+          raise_guest_page_fault(op, gpaddr, vaddr, tinst, effective_mode);
+        }
+      }
       if (!implemented?(ExtensionName::Svnapot)) {
         if ((PTESIZE >= 64) && pte[63] != 0) {
           # N is reserved if Svnapot is not supported
@@ -2127,8 +2133,8 @@ function stage1_page_walk {
       Boolean ss_page = (pte_flags.R == 0) && (pte_flags.W == 1) && (pte_flags.X == 0);
 
       # check if any reserved bits are set
-      # Sv32 has no reserved bits, and Sv39/48/57 all have reserved bits at 60:54
-      if ((VA_SIZE != 32) && (pte[60:54] != 0)) {
+      # Sv32 has no reserved bits, and Sv39/48/57 all have reserved bits at 58:54
+      if ((VA_SIZE != 32) && (pte[58:54] != 0)) {
         raise(page_fault_code, mode(), vaddr);
       }
 
@@ -2156,6 +2162,13 @@ function stage1_page_walk {
         }
       }
 
+      if (!implemented?(ExtensionName::Svrsw60t59b)) {
+        if ((PTESIZE >= 64) && pte[60:59] != 0) {
+          # 60:59 are reserved if Svrsw60t59b is not supported
+          raise (page_fault_code, mode(), vaddr);
+        }
+      }
+
       if (!implemented?(ExtensionName::Svnapot)) {
         if ((PTESIZE >= 64) && (pte[63] != 0)) {
           # N is reserved if Svnapot is not supported