Re: [RFC PATCH 0/6] Support raw event and DT for perf on RISC-V

[Anup Patel]

On Tue, Jun 30, 2020 at 1:34 PM Zong Li <zong.li@xxxxxxxxxx> wrote:
>
> On Tue, Jun 30, 2020 at 3:40 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jun 30, 2020 at 12:07 PM Zong Li <zong.li@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Jun 29, 2020 at 9:23 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Jun 29, 2020 at 6:23 PM Zong Li <zong.li@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Jun 29, 2020 at 4:28 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, Jun 29, 2020 at 11:22 AM Zong Li <zong.li@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Mon, Jun 29, 2020 at 12:53 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On Mon, Jun 29, 2020 at 8:49 AM Zong Li <zong.li@xxxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > This patch set adds raw event support on RISC-V. In addition, we
> > > > > > > > > introduce the DT mechanism to make our perf more generic and common.
> > > > > > > > >
> > > > > > > > > Currently, we set the hardware events by writing the mhpmeventN CSRs, it
> > > > > > > > > would raise an illegal instruction exception and trap into m-mode to
> > > > > > > > > emulate event selector CSRs access. It doesn't make sense because we
> > > > > > > > > shouldn't write the m-mode CSRs in s-mode. Ideally, we should set event
> > > > > > > > > selector through standard SBI call or the shadow CSRs of s-mode. We have
> > > > > > > > > prepared a proposal of a new SBI extension, called "PMU SBI extension",
> > > > > > > > > but we also discussing the feasibility of accessing these PMU CSRs on
> > > > > > > > > s-mode at the same time, such as delegation mechanism, so I was
> > > > > > > > > wondering if we could use SBI calls first and make the PMU SBI extension
> > > > > > > > > as legacy when s-mode access mechanism is accepted by Foundation? or
> > > > > > > > > keep the current situation to see what would happen in the future.
> > > > > > > > >
> > > > > > > > > This patch set also introduces the DT mechanism, we don't want to add too
> > > > > > > > > much platform-dependency code in perf like other architectures, so we
> > > > > > > > > put the mapping of generic hardware events to DT, then we can easy to
> > > > > > > > > transfer generic hardware events to vendor's own hardware events without
> > > > > > > > > any platfrom-dependency stuff in our perf.
> > > > > > > >
> > > > > > > > Please re-write this series to have RISC-V PMU driver as a regular
> > > > > > > > platform driver as drivers/perf/riscv_pmu.c.
> > > > > > > >
> > > > > > > > The PMU related sources will have to be removed from arch/riscv.
> > > > > > > >
> > > > > > > > Based on implementation of final drivers/perf/riscv_pmu.c we will
> > > > > > > > come-up with drivers/perf/riscv_sbi_pmu.c driver for SBI perf counters.
> > > > > > > >
> > > > > > >
> > > > > > > There are some different ways to implement perf, and current
> > > > > > > implementation seems to be consensus when perf was introduced at the
> > > > > > > beginning [0][1]. I don't persist to which one, I could change the
> > > > > > > implementation as you mentioned if it is a new consensus one.
> > > > > > >
> > > > > > > [0] https://github.com/riscv/riscv-linux/pull/124#issuecomment-367563910
> > > > > >
> > > > > > I would not recommend taking the original RISC-V linux fork as reference.
> > > > > >
> > > > > > Rather we should study how things are done on other architectures.
> > > > > >
> > > > > > I really appreciate the attempt to make RISC-V PMU driver depend on DT
> > > > > > but if we are going this route then we should maximize the use of Linux
> > > > > > platform driver framework. In fact, whenever possible we should integrate
> > > > > > RISC-V features as platform drivers under the drivers/ directory.
> > > > > >
> > > > >
> > > > > OK, I would change the implementation to platform driver if there is no
> > > > > other voice.
> > > > >
> > > > > > I thought about SBI PMU counters as well. In future, we can easily
> > > > > > expose SBI PMU counters as RAW events in the same RISC-V PMU
> > > > > > driver. The sbi_probe_extension() can be used in RISC-V PMU driver
> > > > > > to check for SBI PMU counters so no special provisions needed in DT
> > > > > > for SBI PMU counters.
> > > > > >
> > > > >
> > > > > I thought about probing raw events by SBI extension too, I'm interested if you
> > > > > have more detail about this.
> > > > >
> > > > > It seems to me that it is a little bit hard to return all events
> > > > > through one SBI call,
> > > > > so I thought we could map the generic hardware events and maintain their own
> > > > > raw events by each platform in OpenSBI. But eventually, I thought the
> > > > > DT mechanism
> > > > > is more clear and easy than that. Let me know if you have any ideas about
> > > > > probe function. Thanks.
> > > >
> > > > We can design SBI calls such that no SBI call is required to read
> > > > the perf counter.
> > > >
> > > > The sbi_probe_extension() will only be used to check whether
> > > > underlying SBI implementation supports SBI PMU extension.
> > > >
> > > > As-per my initial thoughts, we can potentially have the following SBI calls:
> > > >
> > > > 1. SBI_PMU_NUM_COUNTERS
> > > >     This call will return the number of SBI PMU counters
> > > > 2. SBI_PMU_COUNTER_DESCRIBE
> > > >    This call takes two parameters: 1) physical address 2) counter index
> > > >     It will write the description of SBI PMU counter at specified
> > > > physical address.
> > > >     The details of the SBI PMU counter will include name, type, etc
> > >
> > > The main things are that we need to pass the information of raw events
> > > and the information of mapping of generic hardware events. Maybe
> > > this information could be passed by this SBI call.
> > >
> > > > 3. SBI_PMU_COUNTER_START
> > > >     This call takes two parameters: 1) physical address 2) counter index
> > > >     It will inform SBI implementation to start counting specified counter on the
> > > >     calling HART. The counter value will be written to the specified physical
> > > >     address whenever it changes.
> > >
> > > I would prefer to read the counter directly on s-mode. Spec already defines the
> > > mechanism to allow that. But this way would still work if we couldn't
> > > read counters
> > > on s-mode.
> >
> > The SBI PMU counters have nothing to do with RISC-V PMU counters because
> > these are counters provided by SBI implementation.
> >
> > All-in-all, we have three types of counters:
> > 1. PMU counters defined by RISC-V privilege spec. These are TIME,
> > INSRET, and CYCLE CSRs.
> > 2. Implementation specific counters accessed via HPMCOUNTER CSRs.
> > 3. SBI PMU counters for traps taken and processed by M-mode runtime
> > firmware. Examples: number of misaligned load/store, number of illegal
> > instructions, number of SBI RFENCE calls, number of SBI IPI calls, etc.
> >
> > The DT based RISC-V PMU platform driver being discussed in this email
> > thread only addresses points 1) and 2) above.
> >
>
> OK, sounds good, I misunderstood your ideas, I mixed the 2) and 3)
> and see them as the same thing. Many thanks for the clear explanation.

Cool, we are on the same page till here.

>
> > For point 3) above, we need to first define SBI PMU extension. Once SBI
> > PMU extension is defined, we can have separate SBI PMU driver in Linux
> > or extend RISC-V PMU driver to register additonal counters based on
> > SBI PMU extension.
> >
> > I never suggested  to access RISC-V HPMCOUNTER CSRs via SBI calls
> > so DT based RISC-V PMU platform driver (for 1) and 2) above) is good
> > to have. The SBI PMU extension is a separate topic.
> >
> > >
> > > > 4. SBI_PMU_COUNTER_STOP
> > > >     This call takes one parameter: 1) counter index
> > > >     It will inform SBI implementation to stop counting specified counters on
> > > >     the calling HART.
> > > >
> > > > The above calls are generic enough to support any number of counters
> > > > and we don't need any SBI call to read the counter. We can also assume
> > > > all counters to be of fixed 64bit width. In fact, even Hypervisors can support
> > > > it's own SBI PMU counters with SBI PMU extension.
> > > >
> > > > We still need to think more about the above calls because above SBI
> > > > calls are just initial ideas.
> > > >
> > >
> > > We also need a SBI call to set the event selector to specify which event
> > > is monitored.
> >
> > SBI_PMU_COUNTER_START will do that.
>
> I'm not sure whether this SBI call is only for SBI PMU counter and
> it's own events.
> For 2), it needs one SBI call to set the events, we just set the event selector
> by writing m-mode CSRs on s-mode now. If this SBI call could serve 2)
> and 3) both,
> we don't need another SBI call.

Can you elaborate more ??

Is the SBI call for 2) needed to enable/disable counters in MCOUNTEREN CSR ?

Currently, OpenSBI enables all counters by default but I see the need
to enable/disable HPMCOUNTER on-demand from perf event start/stop.

I hope we don't need any other implementation specific CSR to be programmed
for enabling/disabling counters on SiFive Unleashed ??

Regards,
Anup

>
> >
> > >
> > > > Maybe you can refine the above ideas and send a proposal to the
> > > > UnixPlatformSpec mailing list ??
> > > >
> > >
> > > Ok, let us talk about the details in that.
> >
> > Regards,
> > Anup
> >
> > >
> > >
> > > > Regards,
> > > > Anup
> > > >
> > > > >
> > > > > > Also, the RISC-V PMU driver can be implemented such that it will
> > > > > > work for RV32, RV64, NoMMU RV32, and NoMMU RV64.
> > > > > >
> > > > > > Regards,
> > > > > > Anup
> > > > > >
> > > > > > > [1] https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/f19TmCNP6yA
> > > > > > >
> > > > > > > > Regards,
> > > > > > > > Anup
> > > > > > > >
> > > > > > > > >
> > > > > > > > > Zong Li (6):
> > > > > > > > >   dt-bindings: riscv: Add YAML documentation for PMU
> > > > > > > > >   riscv: dts: sifive: Add DT support for PMU
> > > > > > > > >   riscv: add definition of hpmcounter CSRs
> > > > > > > > >   riscv: perf: Add raw event support
> > > > > > > > >   riscv: perf: introduce DT mechanism
> > > > > > > > >   riscv: remove PMU menu of Kconfig
> > > > > > > > >
> > > > > > > > >  .../devicetree/bindings/riscv/pmu.yaml        |  59 +++
> > > > > > > > >  arch/riscv/Kconfig                            |  13 -
> > > > > > > > >  arch/riscv/boot/dts/sifive/fu540-c000.dtsi    |  13 +
> > > > > > > > >  arch/riscv/include/asm/csr.h                  |  58 +++
> > > > > > > > >  arch/riscv/include/asm/perf_event.h           | 100 ++--
> > > > > > > > >  arch/riscv/kernel/Makefile                    |   2 +-
> > > > > > > > >  arch/riscv/kernel/perf_event.c                | 471 +++++++++++-------
> > > > > > > > >  7 files changed, 471 insertions(+), 245 deletions(-)
> > > > > > > > >  create mode 100644 Documentation/devicetree/bindings/riscv/pmu.yaml
> > > > > > > > >
> > > > > > > > > --
> > > > > > > > > 2.27.0
> > > > > > > > >