Re: [PATCH 0/4] riscv: Allow userspace to directly access perf counters

[Atish Patra]

On Fri, Apr 14, 2023 at 2:40 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Atish Patra
> > Sent: 13 April 2023 20:18
> >
> > On Thu, Apr 13, 2023 at 9:47 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> > >
> > > riscv used to allow direct access to cycle/time/instret counters,
> > > bypassing the perf framework, this patchset intends to allow the user to
> > > mmap any counter when accessed through perf. But we can't break the
> > > existing behaviour so we introduce a sysctl perf_user_access like arm64
> > > does, which defaults to the legacy mode described above.
> > >
> >
> > It would be good provide additional direction for user space packages:
> >
> > The legacy behavior is supported for now in order to avoid breaking
> > existing software.
> > However, reading counters directly without perf interaction may
> > provide incorrect values which
> > the userspace software must avoid. We are hoping that the user space
> > packages which
> > read the cycle/instret directly, will move to the proper interface
> > eventually if they actually need it.
> > Most of the users are supposed to read "time" instead of "cycle" if
> > they intend to read timestamps.
>
> If you are trying to measure the performance of short code
> fragments then you need pretty much raw access directly to
> the cycle/clock count register.
>
> I've done this on x86 to compare the actual cycle times
> of different implementations of the IP checksum loop
> (and compare them to the theoretical limit).
> The perf framework just added far too much latency,
> only directly reading the cpu registers gave anything
> like reliable (and consistent) answers.
>

This series allows direct access to the counters once configured
through the perf.
Earlier the cycle/instret counters are directly exposed to the
userspace without kernel/perf frameworking knowing
when/which user space application is reading it. That has security implications.

With this series applied, the user space application just needs to
configure the event (cycle/instret) through perf syscall.
Once configured, the userspace application can find out the counter
information from the mmap & directly
read the counter. There is no latency while reading the counters.

This mechanism allows stop/clear the counters when the requesting task
is not running. It also takes care of context switching
which may result in invalid values as you mentioned below. This is
nothing new and all other arch (x86, ARM64) allow user space
counter read through the same mechanism.

Here is the relevant upstream discussion:
https://lore.kernel.org/lkml/Y7wLa7I2hlz3rKw%2F@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/

ARM64:
https://docs.kernel.org/arm64/perf.html?highlight=perf_user_access#perf-userspace-pmu-hardware-counter-access

example usage in x86:
https://github.com/andikleen/pmu-tools/blob/master/jevents/rdpmc.c

> Clearly process switches (especially cpu migrations) cause
> problems, but they are obviously invalid values and can
> be ignored.
>
> So while a lot of uses may be 'happy' with the values the
> perf framework gives, sometimes you do need to directly
> read the relevant registers.
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)



-- 
Regards,
Atish