Re: Proposal v2: SBI PMU Extension


Zong Li
 

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?



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


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