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

[Bin Meng]

On Wed, Jun 26, 2019 at 4:23 AM Jonathan Behrens <address@hidden> 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