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riscv / riscv-j-extension

Working Draft of the RISC-V J Extension Specification
https://jira.riscv.org/browse/RVG-128
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Zjpm: Restrictions on N for higher privilege with respect to lower privilege #24

Closed deepak0414 closed 11 months ago

deepak0414 commented 1 year ago

Spec doesn't put restrictions on values of N that can be programmed by lesser privileged.

As an example if S mode has configured a value of N = 18 for itself then U mode shouldn't be able to program anything > 18 for user pointers.

This is quite obvious and no implementation should allow this to happen. But calling it out in spec will be be good.

martinmaas commented 1 year ago

Addressed in fd7220807e1405ba9b5de38d9d4ac26e73312128 (Section 2.5)

deepak0414 commented 1 year ago

Section 2.5 says

For example, an operating system's ABI may mandate that (e.g.,) user mode should never mask more bits than kernel mode

This means that in order to enforce this kernel mode would never allow user to change it's N. If that's the case, I believe user mode's config (upm) will always be dictated by kernel, rendering upm not very useful for all practical purposes.

martinmaas commented 11 months ago

Since the latest version of the spec no longer supports delegation and only supports N = 16 across all privilege modes, I think this issue should be addressed now.