target/mips: Add a placeholder for R5900 SQ, handle user mode RDHWR

Add placeholder for SQ instruction, handle RDHWR.

Backports commit bb41e74b66a8879ba5c23db145039faa27df5766 from qemu

qemu/target/mips/translate.c

@@ -24570,6 +24570,53 @@ static void decode_opc_special3_legacy(CPUMIPSState *env, DisasContext *ctx)
     }
 }
 
+static void gen_tx79_sq(DisasContext *ctx, int base, int rt, int offset)
+{
+    generate_exception_end(ctx, EXCP_RI);    /* TODO: TX79_SQ */
+}
+
+/*
+ * The TX79-specific instruction Store Quadword
+ *
+ * +--------+-------+-------+------------------------+
+ * | 011111 |  base |   rt  |           offset       | SQ
+ * +--------+-------+-------+------------------------+
+ *      6       5       5                 16
+ *
+ * has the same opcode as the Read Hardware Register instruction
+ *
+ * +--------+-------+-------+-------+-------+--------+
+ * | 011111 | 00000 |   rt  |   rd  | 00000 | 111011 | RDHWR
+ * +--------+-------+-------+-------+-------+--------+
+ *      6       5       5       5       5        6
+ *
+ * that is required, trapped and emulated by the Linux kernel. However, all
+ * RDHWR encodings yield address error exceptions on the TX79 since the SQ
+ * offset is odd. Therefore all valid SQ instructions can execute normally.
+ * In user mode, QEMU must verify the upper and lower 11 bits to distinguish
+ * between SQ and RDHWR, as the Linux kernel does.
+ */
+static void decode_tx79_sq(CPUMIPSState *env, DisasContext *ctx)
+{
+    int base = extract32(ctx->opcode, 21, 5);
+    int rt = extract32(ctx->opcode, 16, 5);
+    int offset = extract32(ctx->opcode, 0, 16);
+
+#ifdef CONFIG_USER_ONLY
+    uint32_t op1 = MASK_SPECIAL3(ctx->opcode);
+    uint32_t op2 = extract32(ctx->opcode, 6, 5);
+
+    if (base == 0 && op2 == 0 && op1 == OPC_RDHWR) {
+        int rd = extract32(ctx->opcode, 11, 5);
+
+        gen_rdhwr(ctx, rt, rd, 0);
+        return;
+    }
+#endif
+
+    gen_tx79_sq(ctx, base, rt, offset);
+}
+
 static void decode_opc_special3(CPUMIPSState *env, DisasContext *ctx)
 {
     TCGContext *tcg_ctx = env->uc->tcg_ctx;