Re: [RFC 4/9] drivers/perf: riscv: Read upper bits of a firmware counter

[Anup Patel]

On Thu, Dec 7, 2023 at 6:03 PM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 04, 2023 at 06:43:05PM -0800, Atish Patra wrote:
> > SBI v2.0 introduced a explicit function to read the upper bits
> > for any firmwar counter width that is longer than XLEN. Currently,
> > this is only applicable for RV32 where firmware counter can be
> > 64 bit.
>
> The v2.0 spec explicitly says that this function returns the upper
> 32 bits of the counter for rv32 and will always return 0 for rv64
> or higher. The commit message here seems overly generic compared to
> the actual definition in the spec, and makes it seem like it could
> be used with a 128 bit counter on rv64 to get the upper 64 bits.
>
> I tried to think about what "generic" situation the commit message
> had been written for, but the things I came up with would all require
> changes to the spec to define behaviour for FID #5 and/or FID #1, so
> in the end I couldn't figure out the rationale behind the non-committal
> wording used here.
>
> >
> > Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> > ---
> >  drivers/perf/riscv_pmu_sbi.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> > index 40a335350d08..1c9049e6b574 100644
> > --- a/drivers/perf/riscv_pmu_sbi.c
> > +++ b/drivers/perf/riscv_pmu_sbi.c
> > @@ -490,16 +490,23 @@ static u64 pmu_sbi_ctr_read(struct perf_event *event)
> >       struct hw_perf_event *hwc = &event->hw;
> >       int idx = hwc->idx;
> >       struct sbiret ret;
> > -     union sbi_pmu_ctr_info info;
> >       u64 val = 0;
> > +     union sbi_pmu_ctr_info info = pmu_ctr_list[idx];
> >
> >       if (pmu_sbi_is_fw_event(event)) {
> >               ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_FW_READ,
> >                               hwc->idx, 0, 0, 0, 0, 0);
> >               if (!ret.error)
> >                       val = ret.value;
> > +#if defined(CONFIG_32BIT)
>
> Why is this not IS_ENABLED()? The code below uses one. You could then
> fold it into the if statement below.
>
> > +             if (sbi_v2_available && info.width >= 32) {
>
>  >= 32? I know it is from the spec, but why does the spec define it as
>  "One less than number of bits in CSR"? Saving bits in the structure I
>  guess?

Yes, it is for using fewer bits in counter_info.

The maximum width of a HW counter is 64 bits. The absolute value 64
requires 7 bits in counter_info whereas absolute value 63 requires 6 bits
in counter_info. Also, a HW counter if it exists will have at least 1 bit
implemented otherwise the HW counter does not exist.

Regards,
Anup

>
> > +                     ret = sbi_ecall(SBI_EXT_PMU, SBI_EXT_PMU_COUNTER_FW_READ_HI,
> > +                                     hwc->idx, 0, 0, 0, 0, 0);
>
> > +                     if (!ret.error)
> > +                             val = val | ((u64)ret.value << 32);
>
> If the first ecall fails but the second one doesn't won't we corrupt
> val by only setting the upper bits? If returning val == 0 is the thing
> to do in the error case (which it is in the existing code) should the
> first `if (!ret.error)` become `if (ret.error)` -> `return 0`?
>
>
> > +                             val = val | ((u64)ret.value << 32);
>
> Also, |= ?
>
> Cheers,
> Conor.
>
> > +             }
> > +#endif
> >       } else {
> > -             info = pmu_ctr_list[idx];
> >               val = riscv_pmu_ctr_read_csr(info.csr);
> >               if (IS_ENABLED(CONFIG_32BIT))
> >                       val = ((u64)riscv_pmu_ctr_read_csr(info.csr + 0x80)) << 31 | val;
> > --
> > 2.34.1
> >