Architecture Review Committee meeting minutes, Jan 3

John Hauser

Pointer-masking extension

- The ARC proposed a meeting with the J Extension Task Group to discuss
some specifics of the latest draft proposal.


- Discussed the ongoing rework of the IOMMU draft specification's
labeling of stages of address translation. The ARC expressed
a preference for renaming IOMMU "S-stage" and "VS-stage"
to "first-stage"/"stage 1", and renaming IOMMU "G-stage" to
"second-stage"/"stage 2". The committee also believes the IOMMU
document will need commentary about mapping a hart's address
translation stages (single-stage, VS-stage, and G-stage) to the
first- and second-stage translations supported by an IOMMU.

RAS exception codes and interrupt numbers

- Reviewed earlier debates about the need for one or more distinct
exception codes to indicate "corrupt data" when an explicit or
implicit memory read does not complete successfully due to an
uncorrected memory error. Re-affirmed earlier conclusions that:
(a) corrupt data errors should not be reported as access faults, and
(b) a single "corrupt data" exception code appears to be adequate,
because systems are expected to provide any additional information
about memory errors through a separate "RAS" interface.

Consequently, the ARC is inviting a proposal for a fast-track
extension---probably named Ssecorrupt---that defines exception
code 32 to mean "corrupt data" and specifies the circumstances under
which a synchronous exception trap with this code will be taken.
(For RV32, a new high-half CSR, medelegh, is also needed, although
an implementation may make it simply read-only zero.)

- Resumed a debate about the justification for a possible fast-track
extension to extend the AIA (Advanced Interrupt Architecture) by
defining two local interrupt numbers to use for RAS interrupts, and
nothing else. No firm conclusion reached on this topic.