The SiFive U74-MC 21G1.02.00 is used inside the StarFive JH7110 SoC. I could not find a publicly available PDF document for “SiFive U74-MC Core Complex Manual 21G1.02.00”, so I am using a “SiFive U74-MC Core Complex Manual 21G1.01.00” document for my reference and in Chapter 7 “Custom Instructions and CSRs” it lists the following custom instructions:
7.1 CFLUSH.D.L1 (Only available in M-mode)
7.2 CDISCARD.D.L1 (Only available in M-mode)
7.3 CEASE (Privileged instruction only available in M-mode)
7.4 PAUSE
7.5 Branch Prediction Mode CSR (M-mode CSR)
7.6 SiFive Feature Disable CSR (M-mode CSR)
Any mention of “x” would probably be in reference to the above four custom SiFive instructions and the two custom Control and Status Registers.
There is also an optional “SiFive Custom Instruction Extension (SCIE)” which may or may not be present in the JH7110. This would typically be added for 1 to 2 cycle custom instructions.