For the record, kmemcheck requirements for an instruction decoder are these:
For any instruction with memory operands, we need to know which are
the operands (so for movl %eax, (%ebx) we need to combine the
instruction with a struct pt_regs to get the actual address
dereferenced, i.e. the contents of %ebx), and their sizes (for movzbl,
the source operand is 8 bits, destination operand is 32 bits). For
things like movsb, we need to be able to get both %esi and %edi.
mmiotrace additionally needs to know what the actual values
read/written were, for instructions that read/write to memory (again,
combined with a struct pt_regs).
Maybe this doesn't really say much, since this is what a generic
instruction decoder would be able to do anyway. But kmemcheck and
mmiotrace both have very special-purpose decoders. I don't really know
what other decoders look like, but what I would wish for is this: Some
macros for iterating the operands, where each operand has a type (e.g.
input (for reads), output (for writes), target (for jumps), immediate
address, immediate value, etc.), a size (in bits), and a way to
evaluate the operand. So eval(op, regs) for op=%eax, it will return
regs->eax; for op=4(%eax), it will return regs->eax + 4; for op=4 it
will return 4, etc.