Re: [PATCH 1/2] target/riscv: fence.i: update decode pattern

[Philipp Tomsich]

On Fri, 12 Aug 2022 at 16:01, Andrew Jones <ajones@ventanamicro.com> wrote:
>
> > Update the decode pattern to reflect the specification.
>
> I got hung-up on this for a bit since there isn't any "must-be-0" fields,

Please refer to '“Zifencei” Instruction-Fetch Fence, Version 2.0' in
the specification.
The encoding diagram clearly states 0 for imm[11:0], 0 for rs1 and 0 for rd.

However, there is an explanatory paragraph below (unfortunately, it is
not clear whether this is normative or informative):
> The unused fields in the FENCE.I instruction, imm[11:0], rs1, and rd, are 
> reserved for finer-grain fences in future extensions. For forward 
> compatibility, base implementations shall ignore these fields, and standard 
> software shall zero these fields.

Strictly speaking, this patch may be too restrictive (it violates the
"for forward-compatibility" part — which I consider informative only,
though).

Thanks,
Philipp.