Re: v8.1M cpu emulation and target-arm feature-identification strategy

[Richard Henderson]

On 8/5/20 4:08 AM, Peter Maydell wrote:
> Mostly recently we've been aiming for QEMU emulation code in
> target/arm to use ID register fields to determine whether a
> feature is present or not (the isar_feature_* functions) rather
> than the old style of defining ARM_FEATURE_* flags. This seems to
> be working out well for A-profile. However, for v8.1M there are
> a small handful of minor behaviour differences which don't have an
> associated ID register field, but which are instead in the spec
> and pseudocode just called out as "if this is a v8.1M CPU".
> (The major v8.1M new features do have ID register fields.)
> 
> I can think of two ways to handle this:
>  (1) define an ARM_FEATURE_V81M flag
>  (2) define an isar_feature_aa32_v81m() function which under the
>      hood is actually testing for a specific feature which happens
>      to be known to be always present in v8.1M, like low-overhead-branches
>      (ie ID_ISAR0.CmpBranch >=3)

I think (2) has the potential to be confusing in odd ways.  If there really is
no official flag for this, I think we should use (1).


r~