Architecture Review Committee meeting minutes, Jan 10

John Hauser

Pointer-masking extension

- On Monday, members of the ARC met with the J Extension Task Group to
review the status of the proposed pointer-masking extension.

- It is the sense of the ARC that the current draft strives to solve
multiple separate problems (notably "address sanitizing" for one, and
sandboxing for another) with a small combination of features that all
happen to involve modifying memory addresses. The ARC believes the
proposal would benefit from a clearer enumeration of all the intended
uses, and for each, a review of the possible new hardware that could
help, considered separately from the other use cases. The results of
this exercise might suggest a splitting of the current draft into two
or more smaller extensions, targeting different subsets of use cases
with different hardware. The path to completion and ratification
may also be easier for an initial extension with basic functionality
that supports straightforward use cases (such as HWASAN).

- Regarding claims that pointer-masking may be useful within OS code,
a concern was expressed that masking off the upper bits of pointers
could interfere with the typical convention that a virtual address's
most-significant bit (bit 63 or 31) reflects the privilege for the
addressed memory. Other architectures have encountered similar
issues, and an appropriate solution needs to be found for RISC-V.

- Along similar lines, the ARC was uncertain whether there is enough
experience with some of the proposed use cases when in untranslated
modes of operation (M-mode and Bare translation mode). An initial
extension should probably concentrate on the most well-established
use cases in translated modes of operation. A follow-on extension
can then tackle the specifics for operation in untranslated modes.

- Other feedback will be given to the J Extension TG.

Vector crypto extension

- The proposed draft is believed to still need work to meet RVIA
standards for correctness and clarity. Specific further feedback
is being provided.

This week's committee chairs meeting was recapped and discussed.

There was also a general discussion about steps ARC members need to
take in order for some outstanding fast-track extensions to progress,
such as Zfa, Zimops, and Smrnmi.