Re: Proposal v2: SBI PMU Extension


Zong Li
 

On Wed, Jul 8, 2020 at 2:17 PM Atish Patra <atish.patra@...> wrote:

On Wed, 2020-07-08 at 03:04 +0000, Anup Patel wrote:
Hi Atish,

-----Original Message-----
From: Atish Patra <Atish.Patra@...>
Sent: 08 July 2020 00:44
To: zong.li@...; Anup Patel <Anup.Patel@...>
Cc: andrew@...; tech-unixplatformspec@...;
gfavor@...
Subject: Re: [RISC-V] [tech-unixplatformspec] Proposal v2: SBI PMU
Extension

On Tue, 2020-07-07 at 11:05 +0800, Zong Li wrote:
On Tue, Jul 7, 2020 at 12:21 AM Anup Patel <anup.patel@...>
wrote:

-----Original Message-----
From: Zong Li <zong.li@...>
Sent: 06 July 2020 13:59
To: Anup Patel <Anup.Patel@...>
Cc: tech-unixplatformspec@...; Andrew Waterman
<andrew@...>; Greg Favor <gfavor@...>
Subject: Re: [RISC-V] [tech-unixplatformspec] Proposal v2:
SBI PMU
Extension

On Mon, Jul 6, 2020 at 12:35 AM Anup Patel <
anup.patel@...>
wrote:
Hi All,

We don't have a dedicated RISC-V PMU extension but we do
have
HARDWARE
performance counters such as CYCLE CSR, INSTRET CSR, and
HPMCOUNTER CSRs. A RISC-V implementation can support
monitoring
various HARDWARE events using limited number of HPMCOUNTER
CSRs.
In addition to HARDWARE performance counters, a SBI
implementation (e.g. OpenSBI, Xvisor, KVM, etc) can provide
SOFTWARE counters for events such as number of RFENCEs,
number
of IPIs, number of misaligned load/store instructions,
number of
illegal instructions, etc.

We propose SBI PMU extension which tries to cover CYCLE
CSR,
INSTRET CSR, HPMCOUNTER CSRs and SOFTWARE counters provided
by
SBI
implementation.
To define SBI PMU extension, we first define counter_idx
which
is a logical number assigned to a counter and event_idx
which is
an encoded
Is there more detail about counter_idx? I was wondering that
1.
What is the
ordering of logical numbers for HW and SW counters? I think
that
the logical numbers are assigned by OpenSBI.
Like mentioned here, counter_idx is a logical index for all
available counters (i.e. HARDWARE and SOFTWARE). The SBI
implementation (i.e. OpenSBI, Xvisor RISC-V, or KVM RISC-V) can
assign counter_idx to HARDWARE and SOFTWARE counters in any
order it
likes.

2. How to know the logical number of counter_idx of each HW
and SW
counters from s-mode? I guess that we need to know the
logical
numbers of all counters before we invoke a SBI call.
The SBI_PMU_COUNTER_DESCRIBE call mentioned below will tell us
whether given counter_idx maps to a HARDWARE counter or
SOFTWARE
counter based on CSR_Number info returned by
SBI_PMU_COUNTER_DESCRIBE call.
OK, I assume the logical number of counte_idx is sequential and
started from zero here, so during initialization of s-mode
software,
we could get the total number 'N' of counters by
SBI_PMU_NUM_COUNTERS
first, then loop the N times to identify capability of each
counter.
Does it align your ideas?
That's what my understanding as well. Assigning continous
counter_idx may
put a restriction on M-mode implementation. How about assigning
some
There is not restriction on M-mode runtime firmware in assigning
counter_idx
to various HARDWARE and SOFTWARE counters. In fact, counter_idx being
logical index helps M-mode software to implement a registration
mechanism.

ranges for software vs hardware counters. May be split the hardware
into
different ranges as well based on event_idx.type.
I had done that initially but it will only increase SBI calls because
we will
need separate SBI calls to determine number of HARDWARE and SOFTWARE
counters.
I was suggesting to have fixed ranges for both event types.

Also, this makes things difficult if a RISC-V implementation has non-
standard
implementation specific CSR as HARDWARE counter.
But I agree that it gets tricky with non-standard implementation
specific counters.

This also allows supervisor to know what type of the counter it is
looking at
without parsing the data written by the describe call.
There is no real advantage of knowing type of counter from
counter_idx
over CSR_Number returned by SBI_PMU_COUNTER_DESCRIBE call because
the SBI_PMU_COUNTER_DESCRIBE call will be called only at boot-time
once for each counter and S-mode software can mark counters as
HARDWARE/SOFTWARE at boot-time based on CSR_Number returned
by SBI_PMU_COUNTER_DESCRIBE call.
My concern is that it may increase the booting time.
For example, my current x86 desktop has 1679 counters. If a RISC-V
desktop has those many counters (hopefully one day!! :)), there will
be ~2k SBI calls and memory reads just to get perf working. I guess
there will be even more counters in servers.

Moreover, supervisor OS may choose to configure only few basic perf
counter at boot time and defer configuring everything later depending
on the usecase. Having a continous logical counter_idx may prevent
those kind of optimizations. Correct ?
Based on the optimization as you mentioned, it is good to me if we have SBI
call to get the number of HW and SW counters respectively. If s-mode OS
can know the separating numbers, then s-mode OS can lazy assign and query
counters no matter if the counter_idx is continuous or not. If
counter_idx is started
for HW counters, the start countex_idx of the SW counter is the number of HW
counters.

I would suggest that SBI_PMU_NUM_COUNTER can take a parameter to return
the total number of all counters, the number of SW counters only and
the number of
HW counters only.


number representing the HARDWARE/SOFTWARE event to be
monitored.
The SBI PMU event_idx is a XLEN bits wide number encoded as
follows:
event_idx[XLEN-1:16] = info
event_idx[15:12] = type
event_idx[11:0] = code

If event_idx.type == 0x0 then it is HARDWARE event. For
HARDWARE
event, the event_idx.info is optional and can be passed
zero
whereas the event_idx.code can be one of the following
values:
enum sbi_pmu_hw_id {
SBI_PMU_HW_CPU_CYCLES = 0,
SBI_PMU_HW_INSTRUCTIONS = 1,
SBI_PMU_HW_CACHE_REFERENCES = 2,
SBI_PMU_HW_CACHE_MISSES = 3,
SBI_PMU_HW_BRANCH_INSTRUCTIONS = 4,
SBI_PMU_HW_BRANCH_MISSES = 5,
SBI_PMU_HW_BUS_CYCLES = 6,
SBI_PMU_HW_STALLED_CYCLES_FRONTEND = 7,
SBI_PMU_HW_STALLED_CYCLES_BACKEND = 8,
SBI_PMU_HW_REF_CPU_CYCLES = 9,
SBI_PMU_HW_MAX, /* non-ABI */
};
(NOTE: Same as
<linux_source>/include/uapi/linux/perf_event.h)

If event_idx.type == 0x1 then it is HARDWARE CACHE event.
For
HARDWARE
CACHE event, the event_idx.info is optional and can be
passed
zero whereas the event_idx.code is encoded as follows:
event_idx.code[11:3] = cache_id
event_idx.code[2:1] = op_id
event_idx.code[0:0] = result_id
enum sbi_pmu_hw_cache_id {
SBI_PMU_HW_CACHE_L1D = 0,
SBI_PMU_HW_CACHE_L1I = 1,
SBI_PMU_HW_CACHE_LL = 2,
SBI_PMU_HW_CACHE_DTLB = 3,
SBI_PMU_HW_CACHE_ITLB = 4,
SBI_PMU_HW_CACHE_BPU = 5,
SBI_PMU_HW_CACHE_NODE = 6,
SBI_PMU_HW_CACHE_MAX, /* non-ABI */ }; enum
sbi_pmu_hw_cache_op_id
{
SBI_PMU_HW_CACHE_OP_READ = 0,
SBI_PMU_HW_CACHE_OP_WRITE = 1,
SBI_PMU_HW_CACHE_OP_PREFETCH = 2,
SBI_PMU_HW_CACHE_OP_MAX, /* non-ABI */
};
enum sbi_pmu_hw_cache_op_result_id {
SBI_PMU_HW_CACHE_RESULT_ACCESS = 0,
SBI_PMU_HW_CACHE_RESULT_MISS = 1,
SBI_PMU_HW_CACHE_RESULT_MAX, /* non-ABI */
};
(NOTE: Same as
<linux_source>/include/uapi/linux/perf_event.h)

If event_idx.type == 0x2 then it is HARDWARE RAW event. For
HARDWARE RAW event, both event_idx.info and event_idx.code
are
platform
dependent.
If event_idx.type == 0xf then it is SOFTWARE event. For
SOFTWARE
event, event_idx.info is SBI implementation specific and
event_idx.code can be one of the following:
enum sbi_pmu_sw_id {
SBI_PMU_SW_MISALIGNED_LOAD = 0,
SBI_PMU_SW_MISALIGNED_STORE = 1,
SBI_PMU_SW_ILLEGAL_INSN = 2,
SBI_PMU_SW_LOCAL_SET_TIMER = 3,
SBI_PMU_SW_LOCAL_IPI = 4,
SBI_PMU_SW_LOCAL_FENCE_I = 5,
SBI_PMU_SW_LOCAL_SFENCE_VMA = 6,
SBI_PMU_SW_LOCAL_SFENCE_VMA_ASID = 7,
SBI_PMU_SW_LOCAL_HFENCE_GVMA = 8,
SBI_PMU_SW_LOCAL_HFENCE_GVMA_VMID = 9,
SBI_PMU_SW_LOCAL_HFENCE_VVMA = 10,
SBI_PMU_SW_LOCAL_HFENCE_VVMA_ASID = 11,
SBI_PMU_SW_MAX, /* non-ABI */
};

In future, more events can be defined without breaking ABI
compatibility of SBI calls.

Using definition of counter_idx and event_idx, we can
potentially have the following SBI calls:

1. SBI_PMU_NUM_COUNTERS
This call will return the number of COUNTERs
Is it for the SW counters and we get the number of HW
counters by
DT?
Or does it return the number of HW and SW counters both? If
so,
how to distinguish the number of HW and SW?
This call returns total number of counters (i.e. HARDWARE and
SOFTWARE both)

The other question is that the number of SW counters is
defined by
the core of OpenSBI or platform-dependent?
Number of SW counters are defined by SBI implementation (i.e.
OpenSBI,
Xvisor RISC-V, and KVM RISC-V). Most likely SW counters will
not
include any platform-dependent SW counters although this is
design
choice of SBI implementation.
OK, I got it. It would be enough, thanks.

2. 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 written at
specified
physical address are as follows:
1. Name (64 bytes)
2. CSR_Number (2 bytes)
(CSR_Number <= 0xfff means counter is a RISC-V CSR)
(CSR_Number > 0xfff means counter is a SBI
implementation
counter)
(E.g. CSR_Number == 0xC02 imply HPMCOUNTER2 CSR)
3. CSR_Width (2 bytes)
(Number of CSR bits implemented in HW)
4. Event_Count (2 bytes)
(Number of events in Event_List array)
5. Event_List (2 * Event_Count bytes)
(This is an array of 16bit values where each 16bit
value
is the
supported event_idx.type and event_idx.code
combination)
What is the size we should allocate for this physical
address? In
my understanding, we need to allocate the pages in s-mode
first,
then pass the address of the pages to the second parameter,
but we
don't know the event_counter before we allocate the space for
it,
so it might across the boundary if event_count is very big.
Theoretically, Event_Count cannot be more than 65535.

I think we should have SBI_PMU_NUM_EVENTS calls which will
return
number of events supported by given counter_idx. This will help
S-mode software to determine amount of memory to allocate for
SBI_PMU_COUNTER_DESCRIBE.
Sounds good to me.

3. SBI_PMU_COUNTER_SET_PHYS_ADDR
This call takes two parameters: 1) counter_idx 2)
physical
address
It will set the physical address of memory location
where the
SBI
implementation will write the 64bit SOFTWARE counter.
This
SBI call
is only for counters not mapped to any CSR (i.e. only
for
counters
with CSR_Number > 0xfff).
4. SBI_PMU_COUNTER_START
This call takes two parameters: 1) counter_idx 2)
event_idx
It will inform SBI implementation to configure and
start/enable
specified counter on the calling HART to monitor
specific
event.
This SBI call will fail for counters which are not
present
and
specified event_idx is not supported by the counter.
5. 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
counters
which are
not present.

From above, the RISC-V PMU driver will use most of the SBI
calls
at boot time. Only SBI_PMU_COUNTER_START to be used once
before
using
the counter.
The reading of counter is by reading CSR (for CSR_Number <
0xfff) OR
by reading memory location (for CSR_Offset >= 0xfff). The
counter overflow handling will have to be done in software
by
Linux kernel.

Using the SBI PMU extension, the M-mode runtime firmware
(or
Hypervisors) can provide a standardized view of
HARDWARE/SOFTWARE counters and events to S-mode (or VS-
mode)
software.

The M-mode runtime firmware (OpenSBI) will need to know
following platform dependent information:
1. Possible event_idx values allowed (or supported) by a
HARDWARE
counter (i.e. HPMCOUNTER)
2. Mapping of event_idx for HARDWARE event to HPMEVENT CSR
value
3.
Mapping of event_idx for HARDWARE CACHE event to HPMEVENT
CSR
value 4.
Mapping of event_idx for HARDWARE RAW event to HPMEVENT CSR
value
5.
Additional platform-specific progamming required by any
event_idx

All platform dependent information mentioned above, can be
obtained by M-mode runtime firmware (OpenSBI) from platform
specific code.
The
DT/ACPI can also be used to described 1), 2), 3), and 4)
mentioned above but 5) will always require platform
specific
code.
I would update the next version of DT file to describe the
points
from
1) to 4). Thanks.
As you mentioned before, it would be hard to sync the
platform
specific code with the DT of real use.
I prefer to get 1), 2), 3) and 4) from DT first on each
platform,
and use platform specific code if DT is unavailable. (generic
platform use DT certainly), then we could maximally reduce
the
inconsistency.
It should platform's choice on how it wants to describe
HARDWARE
events and HARDWARE counters. The OpenSBI generic platform will
tend
to use DT based parsing of HARDWARE events and HARDWARE
counters
but
other platform can do things differently.

The S-mode software (i.e. Linux) should not get HARDWARE events
and
HARDWARE counters from DT because DT describes HARDWARE and DT
will
not include SOFTWARE events and SOFTWARE counters.
Also, SOFTWARE events and SOFTWARE counters will change for
given
platform as OpenSBI continues to improve so it will be hard to
keep
the DT in sync.

The best thing for S-mode software would be to depend on one
method
of discovering all counters and supported events which is the
SBI_PMU_COUNTER_DESCRIBE call. In other words, no need for
platform
driver for Linux RISC-V PMU driver instead depend only on
sbi_probe_extension() to detect SBI PMU extension.
OK, make sense.

Regards,
Anup



--
Regards,
Atish
Regards,
Anup
--
Regards,
Atish


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