Hi Palmer, On Tue, Jun 25, 2019 at 5:57 PM Palmer Dabbelt <pal...@sifive.com> wrote: > > On Mon, 24 Jun 2019 16:03:20 PDT (-0700), finte...@gmail.com wrote: > > Apparently my previous message didn't make it out onto the list (sorry > > about all these email glitches!). I've included the message again below. > > Hopefully either a patch like this one or something simpler that just hard > > codes mcounteren.TM to zero (so QEMU is at least conformant) can be merged > > in time for 4.1. > > > > On Fri, Jun 14, 2019 at 8:55 AM Jonathan Behrens <jonat...@fintelia.io> > > wrote: > > > >> I'm not sure that is accurate. Based on the discussion here > >> <https://forums.sifive.com/t/possible-bug-in-counteren-csrs/2123> the > >> HiFive Unleashed actually does support reading the timer CSR from > >> unprivileged modes (from that discussion it does so a little too well... > >> but it should presumably be fixed in later iterations of the processor). > >> And even if no real hardware supported this capability, it still might make > >> sense to provide it in QEMU as an optimization. > > time and cycle are different registers: rdtime should trap, but rdcycle should > work. The hardware traps rdtime into machine mode and emulates it via a > memory > mapped register. The bug in the FU540-C000 appears to be related to the > counter enables, not the presence of the counters.
I don't think rdtime is required to mandatorily trap. Per privileged spec v1.10, chapter 3.1.17 Counter-Enable Registers ([m|h|s]counteren), it says: "When the CY, TM, IR, or HPMn bit in the mcounteren register is clear, attempts to read the cycle, time, instret, or hpmcountern register while executing in S-mode or U-mode will cause an illegal instruction exception." In the same chapter, it also says: "Implementations can convert reads of the time CSR into loads to the memory-mapped mtime register, or hard-wire the TM bits in mxcounteren to 0 and emulate this functionality in M-mode software." So per my understanding when mcounteren.TM is set, rdtime should NOT trap. Regards, Bin
