target/riscv: Raise exceptions on wrs.nto

Implementing wrs.nto to always just return is consistent with the
specification, as the instruction is permitted to terminate the
stall for any reason, but it's not useful for virtualization, where
we'd like the guest to trap to the hypervisor in order to allow
scheduling of the lock holding VCPU. Change to always immediately
raise exceptions when the appropriate conditions are present,
otherwise continue to just return. Note, immediately raising
exceptions is also consistent with the specification since the
time limit that should expire prior to the exception is
implementation-specific.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240424142808.62936-2-ajones@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/helper.h

@@ -132,6 +132,7 @@ DEF_HELPER_6(csrrw_i128, tl, env, int, tl, tl, tl, tl)
 DEF_HELPER_1(sret, tl, env)
 DEF_HELPER_1(mret, tl, env)
 DEF_HELPER_1(wfi, void, env)
+DEF_HELPER_1(wrs_nto, void, env)
 DEF_HELPER_1(tlb_flush, void, env)
 DEF_HELPER_1(tlb_flush_all, void, env)
 /* Native Debug */

target/riscv/insn_trans/trans_rvzawrs.c.inc

@@ -16,7 +16,7 @@
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-static bool trans_wrs(DisasContext *ctx)
+static bool trans_wrs_sto(DisasContext *ctx, arg_wrs_sto *a)
 {
     if (!ctx->cfg_ptr->ext_zawrs) {
         return false;
@@ -40,12 +40,23 @@ static bool trans_wrs(DisasContext *ctx)
     return true;
 }
 
-#define GEN_TRANS_WRS(insn)                                     \
-static bool trans_ ## insn(DisasContext *ctx, arg_ ## insn *a)  \
-{                                                               \
-    (void)a;                                                    \
-    return trans_wrs(ctx);                                      \
-}
+static bool trans_wrs_nto(DisasContext *ctx, arg_wrs_nto *a)
+{
+    if (!ctx->cfg_ptr->ext_zawrs) {
+        return false;
+    }
 
-GEN_TRANS_WRS(wrs_nto)
-GEN_TRANS_WRS(wrs_sto)
+    /*
+     * Depending on the mode of execution, mstatus.TW and hstatus.VTW, wrs.nto
+     * should raise an exception when the implementation-specific bounded time
+     * limit has expired. Our time limit is zero, so we either return
+     * immediately, as does our implementation of wrs.sto, or raise an
+     * exception, as handled by the wrs.nto helper.
+     */
+#ifndef CONFIG_USER_ONLY
+    gen_helper_wrs_nto(tcg_env);
+#endif
+
+    /* We only get here when helper_wrs_nto() doesn't raise an exception. */
+    return trans_wrs_sto(ctx, NULL);
+}

target/riscv/op_helper.c

@@ -380,6 +380,17 @@ void helper_wfi(CPURISCVState *env)
     }
 }
 
+void helper_wrs_nto(CPURISCVState *env)
+{
+    if (env->virt_enabled && (env->priv == PRV_S || env->priv == PRV_U) &&
+        get_field(env->hstatus, HSTATUS_VTW) &&
+        !get_field(env->mstatus, MSTATUS_TW)) {
+        riscv_raise_exception(env, RISCV_EXCP_VIRT_INSTRUCTION_FAULT, GETPC());
+    } else if (env->priv != PRV_M && get_field(env->mstatus, MSTATUS_TW)) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    }
+}
+
 void helper_tlb_flush(CPURISCVState *env)
 {
     CPUState *cs = env_cpu(env);