Re: [Qemu-devel] [PATCH for 4.1 v3] target/riscv: Expose time CSRs when allowed by [m|s]counteren

[Alistair Francis <alistair23@gmail.com>]

On Thu, Jun 27, 2019 at 11:12 PM Palmer Dabbelt <palmer@sifive.com> wrote:
>
> On Thu, 27 Jun 2019 12:56:57 PDT (-0700), alistair23@gmail.com wrote:
> > On Wed, Jun 26, 2019 at 1:25 AM Palmer Dabbelt <palmer@sifive.com> wrote:
> >>
> >> On Tue, 25 Jun 2019 23:58:34 PDT (-0700), bmeng.cn@gmail.com wrote:
> >> > On Wed, Jun 26, 2019 at 4:23 AM Jonathan Behrens <fintelia@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.
>
> WARL is Write Any Read Legal.  Essentially it means that the hardware gets to
> decide what sort of behavior that field has, and is the mechanism for optional
> features in the ISA.  In this case it'd be entirely within spec, specifically:
>
>     Registers mcounteren and scounteren are WARL registers that must be
>     implemented if U-mode and S-mode are implemented. Any of the bits may
>     contain a hardwired value of zero, indicating reads to the corresponding
>     counter will cause an illegal instruction exception when executing in a
>     less-privileged mode.
>
> Taking a patch that matches the out-of-spec FU540 behavior doesn't make any
> sense, I don't want to implement errata in QEMU :)

Ah, I misread your email. That is fine then :)

Alistair

>
> >
> > 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
> >>