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

Tue Jun 30 14:57:27 EDT 2020

On Tue, 2020-06-30 at 17:08 +0530, Anup Patel wrote:
> On Tue, Jun 30, 2020 at 3:48 PM Anup Patel <anup at brainfault.org>
> wrote:
> > On Tue, Jun 30, 2020 at 1:34 PM Zong Li <zong.li at sifive.com> wrote:
> > > On Tue, Jun 30, 2020 at 3:40 PM Anup Patel <anup at brainfault.org>
> > > wrote:
> > > > On Tue, Jun 30, 2020 at 12:07 PM Zong Li <zong.li at sifive.com>
> > > > wrote:
> > > > > On Mon, Jun 29, 2020 at 9:23 PM Anup Patel <
> > > > > anup at brainfault.org> wrote:
> > > > > > On Mon, Jun 29, 2020 at 6:23 PM Zong Li <zong.li at sifive.com
> > > > > > > wrote:
> > > > > > > On Mon, Jun 29, 2020 at 4:28 PM Anup Patel <
> > > > > > > anup at brainfault.org> wrote:
> > > > > > > > On Mon, Jun 29, 2020 at 11:22 AM Zong Li <
> > > > > > > > zong.li at sifive.com> wrote:
> > > > > > > > > On Mon, Jun 29, 2020 at 12:53 PM Anup Patel <
> > > > > > > > > anup at brainfault.org> wrote:
> > > > > > > > > > On Mon, Jun 29, 2020 at 8:49 AM Zong Li <
> > > > > > > > > > zong.li at 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
> 3. counter_idx = 32 or higher are for software counters counters
> provided by SBI implementation
> 

The number of HPMCOUNTER may increase in future. Right ?

How about using a higher starting idx for software counters from SBI
impolementation ?

> 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. 

We still need the information in DT for mapping generic hardware
events. No ?

> The availability of SBI PMU extension can be checked
> using sbi_probe_extension() SBI call.
> 
> Regards,
> Anup
> 
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> linux-riscv mailing list
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-- 
Regards,
Atish