Fix hgatp unsupported mode writes

core/csr_regfile.sv

@@ -1579,10 +1579,11 @@ module csr_regfile
               hgatp[1:0] = 2'b0;
               // only make VMID_LEN - 1 bit stick, that way software can figure out how many VMID bits are supported
               hgatp.vmid = hgatp.vmid & {{(CVA6Cfg.VMIDW - CVA6Cfg.VMID_WIDTH) {1'b0}}, {CVA6Cfg.VMID_WIDTH{1'b1}}};
-              // only update if we actually support this mode
-              if (config_pkg::vm_mode_t'(hgatp.mode) == config_pkg::ModeOff ||
-                            config_pkg::vm_mode_t'(hgatp.mode) == CVA6Cfg.MODE_SV)
-                hgatp_d = hgatp;
+              // Preserve the current mode if the written mode is unsupported.
+              if (config_pkg::vm_mode_t'(hgatp.mode) != config_pkg::ModeOff &&
+                  config_pkg::vm_mode_t'(hgatp.mode) != CVA6Cfg.MODE_SV)
+                hgatp.mode = hgatp_q.mode;
+              hgatp_d = hgatp;
             end
             // changing the mode can have side-effects on address translation (e.g.: other instructions), re-fetch
             // the next instruction by executing a flush

verif/tests/custom/hgatp_warl/hgatp_mode_warl.S

@@ -0,0 +1,55 @@
+#include "riscv_test.h"
+#include "test_macros.h"
+
+.option arch, +h
+
+#define CSR_HGATP               0x680
+#define HGATP_MODE_SV39X4       8
+#define HGATP_MODE_UNSUPPORTED  1
+
+RVTEST_RV64M
+RVTEST_CODE_BEGIN
+
+  li TESTNUM, 1
+
+  # Install an initial legal value:
+  # MODE = Sv39x4
+  # PPN  = 0x100
+  li   t0, HGATP_MODE_SV39X4
+  slli t0, t0, 60
+  ori  t0, t0, 0x100
+
+  csrw CSR_HGATP, t0
+  csrr t1, CSR_HGATP
+
+  bne  t1, t0, fail
+
+  li TESTNUM, 2
+
+  # Write an unsupported MODE with a different PPN.
+  # MODE must be legalized, but PPN must still update.
+  li   t2, HGATP_MODE_UNSUPPORTED
+  slli t2, t2, 60
+  ori  t2, t2, 0x200
+
+  csrw CSR_HGATP, t2
+  csrr t3, CSR_HGATP
+
+  # Expected:
+  # MODE remains Sv39x4
+  # PPN changes to 0x200
+  li   t4, HGATP_MODE_SV39X4
+  slli t4, t4, 60
+  ori  t4, t4, 0x200
+
+  bne  t3, t4, fail
+
+  j pass
+
+  TEST_PASSFAIL
+
+RVTEST_CODE_END
+
+RVTEST_DATA_BEGIN
+  TEST_DATA
+RVTEST_DATA_END

verif/tests/testlist_hgatp_warl.yaml

@@ -0,0 +1,8 @@
+# Regression for unsupported hgatp.MODE WARL handling.
+
+testlist:
+  - test: rv64h-p-hgatp-mode-warl
+    iterations: 1
+    path_var: TESTS_PATH
+    gcc_opts: "-static -mcmodel=medany -fvisibility=hidden -nostdlib -nostartfiles -I<path_var>/riscv-tests/isa/macros/scalar/ -I<path_var>/riscv-tests/env/p/ -I<path_var>/riscv-tests/riscv-target/spike/"
+    asm_tests: <path_var>/custom/hgatp_warl/hgatp_mode_warl.S