Mandating of RVA22 S and U ISA Profiles in OS-A platform specs

Greg Favor


Recently a PR was sent out to remove U and VU mode standardization from the platform spec scope.  Which is sort of right and sort of wrong.

I brought this issue up with Krste and Andrew (especially since this also relates with the coming RVA Profiles).  In short, after significant discussion, the following can be said:
  • Support for the RVA22S64 profile does not directly also require support for the RVA22U64 profile.  Although it does require support for U-mode.

  • BUT, for example, all mandatory ISA extensions in the 'S' profile that are also in the 'U' profile must be supported not only in S-mode but also in U-mode.  This only excludes ISA extensions that are not relevant to U-mode, e.g. the Sstc extension.  Largely, if not completely, this amounts to all Unpriv ISA extensions that are required to be supported in S-mode are also required to be supported in U-mode.  The Profile specs will precisely spell out this and the related matter of "optional supported" extensions.

  • The OS-A platforms should only mandate the RVA22S64 profile (and not the RVA22U64 profile as well).  U/VU-mode platform requirements will flow from or be indirectly inherited via the RVA22S64 profile requirement.

So I suggest that the change to the platform spec should be something like:
The current version of this platform spec targets the standardization of functionality available in S and VS modes (plus the support for U/VU-modes, and the support for ISA extensions in U/VU modes, as implied by the RVA 'S' ISA Profile that is mandated by this spec), and the standardization of the SBI (....).


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