diff --git a/nix/overlay.nix b/nix/overlay.nix
index d6c81428b..e4c8c687a 100644
--- a/nix/overlay.nix
+++ b/nix/overlay.nix
@@ -20,6 +20,8 @@ rec {
   libspike = final.callPackage ./pkgs/libspike.nix { };
   libspike_interfaces = final.callPackage ../difftest/spike_interfaces { };
 
+  sail-riscv-c-model = final.callPackage ./pkgs/sail-riscv-c-model.nix { };
+
   # DynamoCompiler doesn't support python 3.12+ yet
   buddy-mlir = final.callPackage ./pkgs/buddy-mlir.nix { python3 = final.python311; };
 
@@ -37,6 +39,11 @@ rec {
   mill = let jre = final.jdk21; in
     (prev.mill.override { inherit jre; }).overrideAttrs (_: {
       passthru = { inherit jre; };
+      # Using mill without "-i/--interactive" or "--no-server" will run a mill
+      # build server automatically. The build server doesn't inherit or update
+      # environment when user invoke mill at command-line, which is hard for
+      # developers to debug.
+      #
       # --interactive implies --no-server
       postInstall = ''wrapProgram $out/bin/mill --add-flags "--interactive"'';
     });
diff --git a/nix/pkgs/sail-riscv-c-model.nix b/nix/pkgs/sail-riscv-c-model.nix
new file mode 100644
index 000000000..1a385b822
--- /dev/null
+++ b/nix/pkgs/sail-riscv-c-model.nix
@@ -0,0 +1,176 @@
+# https://github.com/NixOS/nixpkgs/blob/nixpkgs-unstable/pkgs/applications/virtualization/sail-riscv/default.nix
+
+{ lib
+, stdenv
+, fetchFromGitHub
+, ocamlPackages
+, ocaml
+, zlib
+, z3
+}:
+
+let
+  ocamlPackages' = ocamlPackages.overrideScope (finalScope: prevScope: {
+    sail = prevScope.sail.overrideAttrs rec {
+      version = "0.18";
+      src = fetchFromGitHub {
+        owner = "rems-project";
+        repo = "sail";
+        rev = version;
+        hash = "sha256-QvVK7KeAvJ/RfJXXYo6xEGEk5iOmVsZbvzW28MHRFic=";
+      };
+
+      buildInputs = [ finalScope.menhirLib ];
+    };
+  });
+in
+stdenv.mkDerivation rec {
+  pname = "sail-riscv-riscv-model";
+  version = "unstable-d36ea53";
+
+  src = fetchFromGitHub {
+    owner = "riscv";
+    repo = pname;
+    rev = "d36ea53742d31ae1199620680f01f1771a4c5f3e";
+    hash = "sha256-CnujXPrZHQuWFR+dFsUibkrggX+ZMdsogdrb5fVP0fE=";
+  };
+
+  nativeBuildInputs = with ocamlPackages'; [ ocamlbuild findlib ocaml z3 sail ];
+  buildInputs = with ocamlPackages'; [ zlib linksem ];
+  strictDeps = true;
+
+  postPatch = ''
+    rm -r prover_snapshots
+    rm -r handwritten_support
+  '';
+
+  sailArgs = lib.escapeShellArgs [
+    "--strict-var"
+    "-dno_cast"
+    "-c_preserve"
+    "_set_Misa_C"
+    "-O"
+    "-Oconstant_fold"
+    "-memo_z3"
+    "-c"
+    "-c_no_main"
+  ];
+
+  sailSrcs = lib.escapeShellArgs [
+    "model/prelude.sail"
+    "model/riscv_xlen32.sail"
+    "model/riscv_flen_D.sail"
+    "model/riscv_vlen.sail"
+    "model/prelude_mem_metadata.sail"
+    "model/prelude_mem.sail"
+    "model/riscv_types_common.sail"
+    "model/riscv_types_ext.sail"
+    "model/riscv_types.sail"
+    "model/riscv_vmem_types.sail"
+    "model/riscv_reg_type.sail"
+    "model/riscv_freg_type.sail"
+    "model/riscv_regs.sail"
+    "model/riscv_pc_access.sail"
+    "model/riscv_sys_regs.sail"
+    "model/riscv_pmp_regs.sail"
+    "model/riscv_pmp_control.sail"
+    "model/riscv_ext_regs.sail"
+    "model/riscv_addr_checks_common.sail"
+    "model/riscv_addr_checks.sail"
+    "model/riscv_misa_ext.sail"
+    "model/riscv_vreg_type.sail"
+    "model/riscv_vext_regs.sail"
+    "model/riscv_csr_map.sail"
+    "model/riscv_vext_control.sail"
+    "model/riscv_next_regs.sail"
+    "model/riscv_sys_exceptions.sail"
+    "model/riscv_sync_exception.sail"
+    "model/riscv_next_control.sail"
+    "model/riscv_softfloat_interface.sail"
+    "model/riscv_fdext_regs.sail"
+    "model/riscv_fdext_control.sail"
+    "model/riscv_csr_ext.sail"
+    "model/riscv_sys_control.sail"
+    "model/riscv_platform.sail"
+    "model/riscv_mem.sail"
+    "model/riscv_vmem_common.sail"
+    "model/riscv_vmem_pte.sail"
+    "model/riscv_vmem_ptw.sail"
+    "model/riscv_vmem_tlb.sail"
+    "model/riscv_vmem.sail"
+    "model/riscv_types_kext.sail"
+    "model/riscv_insts_begin.sail"
+    "model/riscv_insts_base.sail"
+    "model/riscv_insts_aext.sail"
+    "model/riscv_insts_zca.sail"
+    "model/riscv_insts_mext.sail"
+    "model/riscv_insts_zicsr.sail"
+    "model/riscv_insts_next.sail"
+    "model/riscv_insts_hints.sail"
+    "model/riscv_insts_fext.sail"
+    "model/riscv_insts_zcf.sail"
+    "model/riscv_insts_dext.sail"
+    "model/riscv_insts_zcd.sail"
+    "model/riscv_insts_svinval.sail"
+    "model/riscv_insts_zba.sail"
+    "model/riscv_insts_zbb.sail"
+    "model/riscv_insts_zbc.sail"
+    "model/riscv_insts_zbs.sail"
+    "model/riscv_insts_zcb.sail"
+    "model/riscv_insts_zfh.sail"
+    "model/riscv_insts_zfa.sail"
+    "model/riscv_insts_zkn.sail"
+    "model/riscv_insts_zks.sail"
+    "model/riscv_insts_zbkb.sail"
+    "model/riscv_insts_zbkx.sail"
+    "model/riscv_insts_zicond.sail"
+    "model/riscv_insts_vext_utils.sail"
+    "model/riscv_insts_vext_fp_utils.sail"
+    "model/riscv_insts_vext_vset.sail"
+    "model/riscv_insts_vext_arith.sail"
+    "model/riscv_insts_vext_fp.sail"
+    "model/riscv_insts_vext_mem.sail"
+    "model/riscv_insts_vext_mask.sail"
+    "model/riscv_insts_vext_vm.sail"
+    "model/riscv_insts_vext_fp_vm.sail"
+    "model/riscv_insts_vext_red.sail"
+    "model/riscv_insts_vext_fp_red.sail"
+    "model/riscv_insts_zicbom.sail"
+    "model/riscv_insts_zicboz.sail"
+    "model/riscv_jalr_seq.sail"
+    "model/riscv_insts_end.sail"
+    "model/riscv_step_common.sail"
+    "model/riscv_step_ext.sail"
+    "model/riscv_decode_ext.sail"
+    "model/riscv_fetch.sail"
+    "model/riscv_step.sail"
+  ];
+
+  buildPhase = ''
+    runHook preInstall
+
+    mkdir -p build
+    sailPhase="sail $sailArgs $sailSrcs -o build/riscv_model_rv32"
+    echo "$sailPhase"
+    eval "$sailPhase"
+
+    runHook postInstall
+  '';
+
+  installPhase = ''
+    runHook preInstall
+
+    mkdir -p $out/share/
+    cp -v build/riscv_model_rv32.c $out/share/
+
+    runHook postInstall
+  '';
+
+  passthru = {
+    inherit (ocamlPackages') sail;
+  };
+
+  meta = {
+    description = "Generated C source of the sail RISC-V model";
+  };
+}