Why is MTIME M-mode only?

[Indanz]

Since commit 59a08cd, which is part of Split region permissions into M-mode and SU-mode MTIME is M-mode only.

This breaks OSes that read MTIME directly to avoid the overhead of emulated rdtime instructions on hardware that don't have native support for the instruction. The emulated rdtime has huge overhead. MTIME is well defined by the RISC-V standard, so we thought we could rely on it.

Is MTIME off-limits for OS code now, and why? Is that documented somewhere? Could read-only access to the CLINT/MTIME memory region be added back so low overhead timer reads are possible again on all RISC-V platforms?

Please advise how we should proceed.