Make Zcheripurecap depend on Zstid

[francislaus]

One of the biggest advantages of CHERI is compartmentalisation. It seems sensible to make Zcheripurecap depend on the existence of the Zstid extension.

[arichardson]

I agree that compartmentalization is essential for CHERI programming models, but I'm not sure we need that dependency.

IMO this dependency really should be set as part of a "cheri purecap profile" since there is no interdependency between these extensions and in theory you could have Zstid without CHERI or CHERI without Zstid.
I think keeping them separate and just strongly recommending both are implemented (or ideally having a profile) should be sufficient.

I am also not sure how useful this extension is without CHERI - since there is no ASR I can only see it being useful if it's only programmable by the mode above?

[davidchisnall]

Completely agreed (with @arichardson). CHERIoT has a rich compartmentalisation model and does not use Zstid (we may replace out use of MTDC with it in a version rebased on Zcheripurecap) but they’re orthogonal features. Zcheripurecap should remain the minimum set of features for CHERI, not a wishlist of all of the things that we might want.

[PeterRugg]

Note that I'm not sure Zstid without CHERI makes sense. It's only ASR that makes the register read-only, so without CHERI it's just another integer CSR that can be written by anyone in your ring.