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From:   Anup Patel <anup@brainfault.org>
Date:   Tue, 30 Jun 2020 17:08:15 +0530
Message-ID: <CAAhSdy02TpyfzTGWzdqFiyeQNTj-RLk61VJNy5pYVYukqvfvrg@mail.gmail.com>
Subject: Re: [RFC PATCH 0/6] Support raw event and DT for perf on RISC-V
To:     Zong Li <zong.li@sifive.com>
Cc:     Alan Kao <alankao@andestech.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
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On Tue, Jun 30, 2020 at 3:48 PM Anup Patel <anup@brainfault.org> wrote:
>
> On Tue, Jun 30, 2020 at 1:34 PM Zong Li <zong.li@sifive.com> wrote:
> >
> > On Tue, Jun 30, 2020 at 3:40 PM Anup Patel <anup@brainfault.org> wrote:
> > >
> > > On Tue, Jun 30, 2020 at 12:07 PM Zong Li <zong.li@sifive.com> wrote:
> > > >
> > > > On Mon, Jun 29, 2020 at 9:23 PM Anup Patel <anup@brainfault.org> wrote:
> > > > >
> > > > > On Mon, Jun 29, 2020 at 6:23 PM Zong Li <zong.li@sifive.com> wrote:
> > > > > >
> > > > > > On Mon, Jun 29, 2020 at 4:28 PM Anup Patel <anup@brainfault.org> wrote:
> > > > > > >
> > > > > > > On Mon, Jun 29, 2020 at 11:22 AM Zong Li <zong.li@sifive.com> wrote:
> > > > > > > >
> > > > > > > > On Mon, Jun 29, 2020 at 12:53 PM Anup Patel <anup@brainfault.org> wrote:
> > > > > > > > >
> > > > > > > > > On Mon, Jun 29, 2020 at 8:49 AM Zong Li <zong.li@sifive.com> 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 ??
>

Here's the next version of SBI PMU extension, which tries to address both
2) and 3). In other words, it covers all HPMCOUNTER CSRs and software
counters of SBI implementation.

To define SBI PMU extension, we first define counter_idx which is a unique
number assigned to a counter:
1. counter_idx = 0 to 2 are for CYCLE, TIME, and INSTRET
2. counter_idx = 3 to 31 are for HPMCOUNTER CSRs
3. counter_idx = 32 or higher are for software counters provided by
SBI implementation

The counter_idx == 1 (i.e. TIME CSR) is always enabled when underlying
HW implements it. Otherwise it is always disabled.

Based on above definition of counter_idx definition, we can potentially have
the following SBI calls:

1. SBI_PMU_NUM_HPMCOUNTER
    This call will return the number of HPMCOUNTER CSRs
2. SBI_PMU_NUM_SOFTWARE
    This call will return the number of software counters provided by
    SBI implementation
3. SBI_PMU_COUNTER_DESCRIBE
    This call takes two parameters: 1) counter_idx 2) physical address
    It will write the description of SBI PMU counter at specified
physical address.
    The details of the SBI PMU counter will include name, type, width,
events etc
4. SBI_PMU_COUNTER_SET_PHYS_ADDR
    This call takes two parameters: 1) counter_idx 2) physical address
    It will set the physical address where SBI implementation will write
    the software counter. This SBI call is only for software counters (i.e.
    counter_idx >= 32) so it will fail for other counters.
5. SBI_PMU_COUNTER_SELECT_EVENT
    This call takes two parameters: 1) counter_idx 2) event number
    It will select a particular HW event to monitor in a HPMCOUNTER CSR.
    This SBI call is only for HPMCOUNTER CSRs (i.e 3 <= counter_idx <= 31)
6. SBI_PMU_COUNTER_START
   This call takes one parameter: 1) counter_idx
   It will inform SBI implementation to start/enable specified counter on the
   calling HART. This SBI call will fail for counter_idx == 1 and counters
   which are not present.
7. SBI_PMU_COUNTER_STOP
    This call takes one parameter: 1) counter_idx
    It will inform SBI implementation to stop/disable specified counters on
    the calling HART. This SBI call will fail for counter_idx == 1 and counters
    which are not present.

The above described SBI calls can be conveniently implemented in
M-mode runtime firmware (OpenSBI) and various hypervisors (Xvisor, KVM, etc).

We can have a single RISC-V PMU driver using above SBI calls which
can be used natively in HS-mode and Guest/VM in VS-mode. Of course,
we won't need any information to be passed in DT/ACPI for this driver
and it can be under arch/riscv/kernel because without DT/ACPI it can't
be a platform driver. The availability of SBI PMU extension can be checked
using sbi_probe_extension() SBI call.

Regards,
Anup