[saagarjha]

You can just keep sticking prefixes on an instruction to get something longer, no? The processor will refuse to decode it but it's "legal" otherwise.

[jcranmer]

Ostensibly, only one prefix from each group can matter. Although I did notice that XACQUIRE and LOCK are both group 1 prefixes, which makes that statement kind of a lie (but it's an intentional design to make XACQUIRE do nothing on processors that don't support it).

In any case, there's only a finite number of prefixes you can meaningfully stick onto an instruction, and repeating the same prefix will do absolutely nothing.

[woodruffw]

Yep: in that sense the longest structurally valid x86 instruction is unbounded in length, so long as you conform to the group restrictions.