On 8/5/20 1:08 PM, 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)
FWIW finding myself sometime git-grepping 'isar_feature' I'd rather choose (2), even if there is no such v8.1M ID register field. That said, my end-user preference isn't very important compared to the developer/maintainer one. > > Any preferences ? > > thanks > -- PMM >
