target/riscv: Align the AIA model to v1.0 ratified spec

According to the new spec, when vsiselect has a reserved value, attempts
from M-mode or HS-mode to access vsireg, or from VS-mode to access
sireg, should preferably raise an illegal instruction exception.

Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-ID: <20230816061647.600672-1-tommy.wu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/csr.c

@@ -1684,7 +1684,7 @@ static int rmw_iprio(target_ulong xlen,
 static int rmw_xireg(CPURISCVState *env, int csrno, target_ulong *val,
                      target_ulong new_val, target_ulong wr_mask)
 {
-    bool virt;
+    bool virt, isel_reserved;
     uint8_t *iprio;
     int ret = -EINVAL;
     target_ulong priv, isel, vgein;
@@ -1694,6 +1694,7 @@ static int rmw_xireg(CPURISCVState *env, int csrno, target_ulong *val,
 
     /* Decode register details from CSR number */
     virt = false;
+    isel_reserved = false;
     switch (csrno) {
     case CSR_MIREG:
         iprio = env->miprio;
@@ -1738,11 +1739,13 @@ static int rmw_xireg(CPURISCVState *env, int csrno, target_ulong *val,
                                                   riscv_cpu_mxl_bits(env)),
                                     val, new_val, wr_mask);
         }
+    } else {
+        isel_reserved = true;
     }
 
 done:
     if (ret) {
-        return (env->virt_enabled && virt) ?
+        return (env->virt_enabled && virt && !isel_reserved) ?
                RISCV_EXCP_VIRT_INSTRUCTION_FAULT : RISCV_EXCP_ILLEGAL_INST;
     }
     return RISCV_EXCP_NONE;