On Wed, Jun 26, 2019 at 1:25 AM Palmer Dabbelt <pal...@sifive.com> wrote: > > On Tue, 25 Jun 2019 23:58:34 PDT (-0700), bmeng...@gmail.com wrote: > > 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 :) > > Ya, it's an errata. There's a handful of them :) > > >> 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? > > In theory we should have properties to control the behavior of all WARL > fields, > but it's a lot of work. I'd be happy to take a patch for any of them.
Hmmm... We should avoid taking patches that don't adhere to the spec just to match some hardware. In the case that core/popular software doesn't work it probably makes sense, but in general it's probably not the best move. Alistair > > >> 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 >
