diff --git a/target/riscv/translate.c b/target/riscv/translate.c
index 38666ddc91..a719aa6e63 100644
--- a/target/riscv/translate.c
+++ b/target/riscv/translate.c
@@ -1022,6 +1022,14 @@ static uint32_t opcode_at(DisasContextBase *dcbase, target_ulong pc)
 /* Include decoders for factored-out extensions */
 #include "decode-XVentanaCondOps.c.inc"
 
+/* The specification allows for longer insns, but not supported by qemu. */
+#define MAX_INSN_LEN  4
+
+static inline int insn_len(uint16_t first_word)
+{
+    return (first_word & 3) == 3 ? 4 : 2;
+}
+
 static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
 {
     /*
@@ -1037,7 +1045,7 @@ static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
     };
 
     /* Check for compressed insn */
-    if (extract16(opcode, 0, 2) != 3) {
+    if (insn_len(opcode) == 2) {
         if (!has_ext(ctx, RVC)) {
             gen_exception_illegal(ctx);
         } else {