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Re: [Qemu-riscv] [Qemu-devel] [PATCH for 4.1 v3] target/riscv: Expose ti

From: Palmer Dabbelt
Subject: Re: [Qemu-riscv] [Qemu-devel] [PATCH for 4.1 v3] target/riscv: Expose time CSRs when allowed by [m|s]counteren
Date: Wed, 26 Jun 2019 01:25:23 -0700 (PDT)

On Tue, 25 Jun 2019 23:54:06 PDT (-0700), address@hidden wrote:
Hi Palmer,

On Tue, Jun 25, 2019 at 5:57 PM Palmer Dabbelt <address@hidden> wrote:

On Mon, 24 Jun 2019 16:03:20 PDT (-0700), address@hidden 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 <address@hidden>
> 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

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.

Implementations are allowed to implement the instruction, but all the exsting
implementations trap to M-mode.

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