Re: Proposal v2: SBI PMU Extension

Anup Patel

-----Original Message-----
From: tech-unixplatformspec@... <tech-
unixplatformspec@...> On Behalf Of Zong Li
Sent: 09 July 2020 14:42
To: Anup Patel <Anup.Patel@...>
Cc: Brian Grayson <brian.grayson@...>; Atish Patra
<Atish.Patra@...>; andrew@...; tech-
unixplatformspec@...; gfavor@...
Subject: Re: [RISC-V] [tech-unixplatformspec] Proposal v2: SBI PMU

On Thu, Jul 9, 2020 at 4:47 PM Anup Patel <Anup.Patel@...> wrote:

-----Original Message-----
From: Zong Li <>
Sent: 09 July 2020 14:09
To: Anup Patel <Anup.Patel@...>
Cc: Brian Grayson <brian.grayson@...>; Atish Patra
<Atish.Patra@...>; andrew@...; tech-
unixplatformspec@...; gfavor@...
Subject: Re: [RISC-V] [tech-unixplatformspec] Proposal v2: SBI PMU

On Thu, Jul 9, 2020 at 3:57 PM Anup Patel <Anup.Patel@...> wrote:

Based on my previous reply…

To monitor RAW event 0x12345678, user-space perf tool will create
space perf RAW event (i.e. perf_event_attr.type == 4 and
perf_event_attr.config = = 0x12345678). The Linux RISC-V PMU driver
will allocate and map matching HARDWARE counter which supports
specified corrosponding SBI RAW event (event_idx.type = 2,
event_idx.code = 0x678 and = 0x12345). Finally, the
SBI_PMU_COUNTER_START call implemented by OpenSBI will write
0x12345678 (or some platform specific translated value of 0x12345678) to
appropriate mhpmeventX CSR).

(Note: above we assume mhpmcounterX supports monitoring RAW
0x12345678 and OpenSBI is aware of this)
The Linux PMU driver should be aware of this as well, because
SBI_PMU_COUNTER_START takes a parameter for countex_idx which is
by Linux PMU driver.
The SBI_PMU_COUNTER_DESCRIBE call is designed considering aspect. This
call provides list of event_idx supported by a given counter_idx
(including RAW events) so when allocating a counter in event_add() we
can easily find matching counter.
Each element of Event_list consists of event_idx.type and event_idx.code,
how does it present the raw event like above? ( = 0x12345,
event_idx.type = 0x2, event_idx.code = 0x678) It seems to conflict if we have
the raw event 0xXXXXX678 and 0xYYYYY678.
The above break-up of 0x12345678 RAW event is only one of the possible
ways shown as example.

If we have two different RAW events 0xXXXXX678 and 0xYYYYY678 then
platform can show it as:
event_idx.type == 2 and event_idx.code == X and = 0xXXXXX678
event_idx.type == 2 and event_idx.code == Y and = 0xYYYYY678
event_idx.type == 2 and event_idx.code == Z and = 0xXXXXX678
event_idx.type == 2 and event_idx.code == Z and = 0xYYYYY678

Further, the OpenSBI platform support can translate RAW event_idx into
final mhpmeventX CSR value using platform callback or describing it in DT/ACPI
for OpenSBI generic platform.

In other words, we have to distinguish all HARDWARE and SOFTWARE
events of a platform using 16bit event_idx.type and event_idx.code. The is only additional parameter to a event.


This means Linux RISC-V PMU only needs to deal with SBI calls for
handling all types of SBI PMU counters and events. In other words, no
need to parse DT/ACPI for event mappings.




From: Brian Grayson <brian.grayson@...>
Sent: 09 July 2020 12:35
To: Zong Li <>
Cc: Anup Patel <Anup.Patel@...>; Atish Patra
<Atish.Patra@...>; andrew@...;
tech-unixplatformspec@...; gfavor@...
Subject: Re: [RISC-V] [tech-unixplatformspec] Proposal v2: SBI PMU

My question is, let's say I know that putting the value 0x12345678
into the
mhpmevent3 register gets me the event I want, and there is no
support for that event in the SBI spec/API. Will this API allow me
to program such an event, basically bypassing the usual mapping
functionality? perf basically allows you to say "I know this event
number is not one you know about, but it's the value I want placed
directly into the hardware." I want to ensure that the full
capabilities of the hardware will still be accessible through the
SBI spec in some sort of "raw" mode, and I didn't see a way for that
to happen right now. We don't want to restrict users to the lowest
common denominator of functionality.


On Wed, Jul 8, 2020 at 10:27 PM Zong Li <> wrote:

On Thu, Jul 9, 2020 at 1:06 AM Brian Grayson

Would there be a raw style interface to access all the
events, like perf's rNNN support?
Follow this question, in our current proposal, s-mode software
only knows the event_idx, and m-mode firmware takes care of the
question is that s-mode software doesn't seem to understand the
meaning of each event_idx, that means, it just get the array of
all supported event_idx, but couldn't know which one is for what.
This also happened on u-mode program, for rNNN interface,
normally, we should refer to the processor specific documentation
for getting these details, and now, users won't know what value
they should give. Please correct me if I miss something. Thanks.

How would this work on a multicore system -- would the SBI calls
handle the current hart's counters? That seems easiest to deal with.


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