AR (Architecture Review) Committee minutes for 11/29/22

Greg Favor

We (the AR Committee) will be posting minutes of our (roughly) weekly meetings to discuss ISA issues that have been raised recently to the committee's attention, and to review RISC-V arch extensions that have been submitted for official AR (or for prelim review).

Here are this week's minutes.   (Note: As the RISC-V Summit and the holidays approach, this becomes a slower period for new AR requests and submissions, and hence general activity.)

Minutes from AR Committee meeting on 11/22/22
  • IOMMU detailed architecture review.
    • Some discussion about the IOMMU explicitly mandating (what is currently rather implicit) that requests by the IOMMU itself are coherent and consistent within the system (i.e. wrt harts and hart software).
    • Conversely, expecting to support IOMMU operation within an incoherent system would raise questions as to whether the IOMMU might be missing desirable arch features to better support use in such systems (i.e. simply saying that software would use cache maintenance instructions when interacting with the IOMMU may gloss over less obvious rough edges or problematic issues that haven't been consciously surfaced and considered).
  • Zicond.
    • Updates to create a final post-AR spec are in process and should be completed soon.  The name is shortened since other instruction extensions don't have the generic 'ops' suffix.

  • Zjid (I/D Consistency)
    • Discussion about this long outstanding ISA extension in development for a couple of years and thoughts about how to help facilitate things after the new year.
    • While at the same time it appears very unlikely that the current proposed Zjid extension can happen in time to be included in RVA23 ISA profiles.
    • So discussion about possible creation of a "Ziic" (Instruction cache Coherence) extension name for use in RVA profiles.  Which, while Zijd gestates, could be used in conjunction with Zifencei to more officially provide support for self and cross modifying code scenarios in the RVA23 profiles.