On Wed, Jun 26, 2019 at 4:23 AM Jonathan Behrens <finte...@gmail.com> wrote: > > I just did some testing on a HiFive Unleashed board and can confirm what > you are saying. The low 5 bits of both mcounteren and scounteren are > writable (if you try to write 0xFFFFFFFF to them, they'll take on the value > 0x1F) but even with the TM bit set in both mcounteren and scounteren the > rdtime instruction always generates an illegal instruction exception. >
Then I would think the FU540 is not spec complaint :) > Reading through the relevant chapter of the spec, I still think that having > mcounteren.TM be writable but making rdtime unconditionally trap is > non-conformant. If other people feel strongly that rdtime should always Agree. To test hardware (FU540) compatibility in QEMU, maybe we can add a cpu property to allow hard-wiring mcounteren.TM to zero? > require trapping into firmware then the natural change would be to simply > hardwire mcounteren.TM to zero (the value in scounteren wouldn't matter in > that case so it could be left writable). My own (biased) personal feeling > is that this full implementation makes sense at least for the `virt` > machine type because it represents a clear case where deviating from > current hardware enables a performance boost, and would not break > compatibility with any current software: both OpenSBI and BBL try to enable > hardware handling of rdtime when the platform claims to support it. > Regards, Bin
