Re: [PATCH v3 28/60] target/arm: Reorg ARMCPRegInfo type field bits

[Peter Maydell]

On Sun, 17 Apr 2022 at 19:03, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Instead of defining ARM_CP_FLAG_MASK to remove flags,
> define ARM_CP_SPECIAL_MASK to isolate special cases.
> Sort the specials to the low bits. Use an enum.
>
> Make ARM_CP_CONST a special case, and ARM_CP_NOP an alias.

This is a behaviour change -- read of an ARM_CP_NOP register
currently is "do not change the destination GPR", but
read of an ARM_CP_CONST register does change the GPR.
Maybe that's OK, but we do have several r/w NOP register
definitions at the moment, and if we do think it's OK to change
that behaviour we should do that carefully in a separate patch.

Also, currently we do not migrate ARM_CP_NOP registers but
we do migrate ARM_CP_CONST registers (and this is important for
KVM --migrating the constant ID registers is how the check for
"is the destination a different host CPU to the source" works).
So overall I don't think it's safe to squash NOP and CONST,
or to treat CONST as one of the SPECIALs.

thanks
-- PMM