Re: [PATCH 2/3] target/riscv: Support discontinuous PMU counters

[Alistair Francis]

On Wed, Oct 11, 2023 at 7:51 PM Rob Bradford <rbradford@rivosinc.com> wrote:
>
> On Mon, 2023-10-09 at 11:00 -0700, Atish Kumar Patra wrote:
> > On Sun, Oct 8, 2023 at 5:58 PM Alistair Francis
> > <alistair23@gmail.com> wrote:
> > >
> > > On Wed, Oct 4, 2023 at 7:36 PM Rob Bradford
> > > <rbradford@rivosinc.com> wrote:
> > > >
> > > > Hi Atish,
> > > >
> > > > On Tue, 2023-10-03 at 13:25 -0700, Atish Kumar Patra wrote:
> > > > > On Tue, Oct 3, 2023 at 5:51 AM Rob Bradford
> > > > > <rbradford@rivosinc.com>
> > > > > wrote:
> > > > > >
> > > > > > There is no requirement that the enabled counters in the
> > > > > > platform
> > > > > > are
> > > > > > continuously numbered. Add a "pmu-mask" property that, if
> > > > > > specified, can
> > > > > > be used to specify the enabled PMUs. In order to avoid
> > > > > > ambiguity if
> > > > > > "pmu-mask" is specified then "pmu-num" must also match the
> > > > > > number
> > > > > > of
> > > > > > bits set in the mask.
> > > > > >
> > > > > > Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
> > > > > > ---
> > > > > >  target/riscv/cpu.c     |  1 +
> > > > > >  target/riscv/cpu_cfg.h |  1 +
> > > > > >  target/riscv/pmu.c     | 15 +++++++++++++--
> > > > > >  3 files changed, 15 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> > > > > > index 9d79c20c1a..b89b006a76 100644
> > > > > > --- a/target/riscv/cpu.c
> > > > > > +++ b/target/riscv/cpu.c
> > > > > > @@ -1817,6 +1817,7 @@ static void
> > > > > > riscv_cpu_add_misa_properties(Object *cpu_obj)
> > > > > >  static Property riscv_cpu_extensions[] = {
> > > > > >      /* Defaults for standard extensions */
> > > > > >      DEFINE_PROP_UINT8("pmu-num", RISCVCPU, cfg.pmu_num, 16),
> > > > > > +    DEFINE_PROP_UINT32("pmu-mask", RISCVCPU, cfg.pmu_mask,
> > > > > > 0),
> > > > > >      DEFINE_PROP_BOOL("sscofpmf", RISCVCPU, cfg.ext_sscofpmf,
> > > > > > false),
> > > > > >      DEFINE_PROP_BOOL("Zifencei", RISCVCPU, cfg.ext_ifencei,
> > > > > > true),
> > > > > >      DEFINE_PROP_BOOL("Zicsr", RISCVCPU, cfg.ext_icsr, true),
> > > > > > diff --git a/target/riscv/cpu_cfg.h b/target/riscv/cpu_cfg.h
> > > > > > index 0e6a0f245c..40f7d970bc 100644
> > > > > > --- a/target/riscv/cpu_cfg.h
> > > > > > +++ b/target/riscv/cpu_cfg.h
> > > > > > @@ -124,6 +124,7 @@ struct RISCVCPUConfig {
> > > > > >      bool ext_XVentanaCondOps;
> > > > > >
> > > > > >      uint8_t pmu_num;
> > > > > > +    uint32_t pmu_mask;
> > > > > >      char *priv_spec;
> > > > > >      char *user_spec;
> > > > > >      char *bext_spec;
> > > > > > diff --git a/target/riscv/pmu.c b/target/riscv/pmu.c
> > > > > > index 13801ccb78..f97e25a1f6 100644
> > > > > > --- a/target/riscv/pmu.c
> > > > > > +++ b/target/riscv/pmu.c
> > > > > > @@ -437,6 +437,13 @@ int riscv_pmu_setup_timer(CPURISCVState
> > > > > > *env,
> > > > > > uint64_t value, uint32_t ctr_idx)
> > > > > >  void riscv_pmu_init(RISCVCPU *cpu, Error **errp)
> > > > > >  {
> > > > > >      uint8_t pmu_num = cpu->cfg.pmu_num;
> > > > > > +    uint32_t pmu_mask = cpu->cfg.pmu_mask;
> > > > > > +
> > > > > > +    if (pmu_mask && ctpop32(pmu_mask) != pmu_num) {
> > > > > > +        error_setg(errp, "Mismatch between number of enabled
> > > > > > counters in "
> > > > > > +                         "\"pmu-mask\" and \"pmu-num\"");
> > > > > > +        return;
> > > > > > +    }
> > > > > >
> > > > >
> > > > > Is that necessary for the default case? I am thinking of
> > > > > marking
> > > > > pmu-num as deprecated and pmu-mask
> > > > > as the preferred way of doing things as it is more flexible.
> > > > > There is
> > > > > no real benefit carrying both.
> > > > > The default pmu-mask value will change in that case.
> > > > > We can just overwrite pmu-num with ctpop32(pmu_mask) if pmu-
> > > > > mask is
> > > > > available. Thoughts ?
> > > > >
> > > >
> > > > I agree it makes sense to me that there is only one way for the
> > > > user to
> > > > adjust the PMU count. However i'm not sure how we can handle the
> > > > transition if we choose to deprecate "pmu-num".
> > > >
> > > > If we change the default "pmu-mask" to MAKE_32BIT_MASK(3, 16)
> > > > then that
> > > > value in the config will always be set - you propose that we
> > > > overwrite
> > > > "pmu-num" with the popcount of that property. But that will break
> > > > if
> > >
> > > Couldn't we deprecate "pmu-num" and then throw an error if both are
> > > set? Then we can migrate away from "pmu-num"
> > >
> >
> > Yeah. pmu-num should be only available as a command line property and
> > marked deprecated.
> > If only pmu-num is set, it gets converted to a mask and throws a
> > warning
> > that this is a deprecated property.
>
> Is there a way to know the property has been set by the user? I
> couldn't see anything in the API - do we just have to assume that if
> the value is not the default then it has been changed by the user?

You should be able to use riscv_cpu_deprecated_exts as a starting point

Alistair