target/riscv: Add Zicfilp support for Smrnmi

Zicfilp extension introduces the MNPELP (bit 9) in mnstatus.
The MNPELP field holds the previous ELP.

When a RNMI trap is delivered, the MNPELP is set to ELP and ELP set
to NO_LP_EXPECTED. Upon a mnret, if the mnstatus.MNPP holds the
value y, then ELP is set to the value of MNPELP if yLPE is 1;
otherwise, it is set to NO_LP_EXPECTED.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250106054336.1878291-7-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu_bits.h

@@ -613,6 +613,7 @@ typedef enum {
 /* RNMI mnstatus CSR mask */
 #define MNSTATUS_NMIE       0x00000008
 #define MNSTATUS_MNPV       0x00000080
+#define MNSTATUS_MNPELP     0x00000200
 #define MNSTATUS_MNPP       0x00001800
 
 /* VM modes (satp.mode) privileged ISA 1.10 */

target/riscv/cpu_helper.c

@@ -1966,6 +1966,10 @@ void riscv_cpu_do_interrupt(CPUState *cs)
         env->mnepc = env->pc;
         env->pc = env->rnmi_irqvec;
 
+        if (cpu_get_fcfien(env)) {
+            env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPELP, env->elp);
+        }
+
         /* Trapping to M mode, virt is disabled */
         riscv_cpu_set_mode(env, PRV_M, false);
 
@@ -2133,7 +2137,12 @@ void riscv_cpu_do_interrupt(CPUState *cs)
         /* handle the trap in M-mode */
         /* save elp status */
         if (cpu_get_fcfien(env)) {
-            env->mstatus = set_field(env->mstatus, MSTATUS_MPELP, env->elp);
+            if (nnmi_excep) {
+                env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPELP,
+                                          env->elp);
+            } else {
+                env->mstatus = set_field(env->mstatus, MSTATUS_MPELP, env->elp);
+            }
         }
 
         if (riscv_has_ext(env, RVH)) {

target/riscv/op_helper.c

@@ -409,6 +409,15 @@ target_ulong helper_mnret(CPURISCVState *env)
 
     riscv_cpu_set_mode(env, prev_priv, prev_virt);
 
+    /*
+     * If forward cfi enabled for new priv, restore elp status
+     * and clear mnpelp in mnstatus
+     */
+    if (cpu_get_fcfien(env)) {
+        env->elp = get_field(env->mnstatus, MNSTATUS_MNPELP);
+    }
+    env->mnstatus = set_field(env->mnstatus, MNSTATUS_MNPELP, 0);
+
     return retpc;
 }