linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Suzuki Kuruppassery Poulose <suzuki.poulose@arm.com>
Cc: catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	maz@kernel.org, sudeep.holla@arm.com, dietmar.eggemann@arm.com,
	peterz@infradead.org, mingo@redhat.com, ggherdovich@suse.cz,
	vincent.guittot@linaro.org, linux-arm-kernel@lists.infradead.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension
Date: Thu, 30 Jan 2020 16:45:42 +0000	[thread overview]
Message-ID: <20200130164542.GC5208@arm.com> (raw)
In-Reply-To: <c9f80a08-7f0d-59e9-eb90-466b1314e1f1@arm.com>

Hi Suzuki,

On Thursday 30 Jan 2020 at 15:04:27 (+0000), Suzuki Kuruppassery Poulose wrote:
> Hi Ionela,
> 
> On 18/12/2019 18:26, Ionela Voinescu wrote:
> > The activity monitors extension is an optional extension introduced
> > by the ARMv8.4 CPU architecture.
> > 
> > Add initial documentation for the AMUv1 extension:
> >   - arm64/amu.txt: AMUv1 documentation
> >   - arm64/booting.txt: system registers initialisation
> >   - arm64/cpu-feature-registers.txt: visibility to userspace
> 
> We have stopped adding "invisible" fields to the list. So, you
> can drop the changes to cpu-feature-registers.txt.
> 
> > 
> > Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > ---
> >   Documentation/arm64/amu.rst                   | 107 ++++++++++++++++++
> >   Documentation/arm64/booting.rst               |  14 +++
> >   Documentation/arm64/cpu-feature-registers.rst |   2 +
> >   Documentation/arm64/index.rst                 |   1 +
> >   4 files changed, 124 insertions(+)
> >   create mode 100644 Documentation/arm64/amu.rst
> > 
> > diff --git a/Documentation/arm64/amu.rst b/Documentation/arm64/amu.rst
> > new file mode 100644
> > index 000000000000..62a6635522e1
> > --- /dev/null
> > +++ b/Documentation/arm64/amu.rst
> > @@ -0,0 +1,107 @@
> > +=======================================================
> > +Activity Monitors Unit (AMU) extension in AArch64 Linux
> > +=======================================================
> > +
> > +Author: Ionela Voinescu <ionela.voinescu@arm.com>
> > +
> > +Date: 2019-09-10
> > +
> > +This document briefly describes the provision of Activity Monitors Unit
> > +support in AArch64 Linux.
> > +
> > +
> > +Architecture overview
> > +---------------------
> > +
> > +The activity monitors extension is an optional extension introduced by the
> > +ARMv8.4 CPU architecture.
> > +
> > +The activity monitors unit, implemented in each CPU, provides performance
> > +counters intended for system management use. The AMU extension provides a
> > +system register interface to the counter registers and also supports an
> > +optional external memory-mapped interface.
> > +
> > +Version 1 of the Activity Monitors architecture implements a counter group
> > +of four fixed and architecturally defined 64-bit event counters.
> > +  - CPU cycle counter: increments at the frequency of the CPU.
> > +  - Constant counter: increments at the fixed frequency of the system
> > +    clock.
> > +  - Instructions retired: increments with every architecturally executed
> > +    instruction.
> > +  - Memory stall cycles: counts instruction dispatch stall cycles caused by
> > +    misses in the last level cache within the clock domain.
> > +
> > +When in WFI or WFE these counters do not increment.
> > +
> > +The Activity Monitors architecture provides space for up to 16 architected
> > +event counters. Future versions of the architecture may use this space to
> > +implement additional architected event counters.
> > +
> > +Additionally, version 1 implements a counter group of up to 16 auxiliary
> > +64-bit event counters.
> > +
> > +On cold reset all counters reset to 0.
> > +
> > +
> > +Basic support
> > +-------------
> > +
> > +The kernel can safely run a mix of CPUs with and without support for the
> > +activity monitors extension.
> 
> 
>  Therefore, when CONFIG_ARM64_AMU_EXTN is
> > +selected we unconditionally enable the capability to allow any late CPU
> > +(secondary or hotplugged) to detect and use the feature.
> > +
> > +When the feature is detected on a CPU, a per-CPU variable (amu_feat) is
> > +set, but this does not guarantee the correct functionality of the
> > +counters, only the presence of the extension.
> 
> nit: I would rather omit the implementation details (esp variable names)
> in the documentation. It may become a pain to keep this in sync with the
> code changes. You could simply mention, "we keep track of the availability
> of the feature" per CPU. If someone wants to figure out
> how, they can always read the code.
> 
> > +
> > +Firmware (code running at higher exception levels, e.g. arm-tf) support is
> > +needed to:
> > + - Enable access for lower exception levels (EL2 and EL1) to the AMU
> > +   registers.
> > + - Enable the counters. If not enabled these will read as 0.
> > + - Save/restore the counters before/after the CPU is being put/brought up
> > +   from the 'off' power state.
> > +
> > +When using kernels that have this configuration enabled but boot with
> > +broken firmware the user may experience panics or lockups when accessing
> > +the counter registers. Even if these symptoms are not observed, the
> > +values returned by the register reads might not correctly reflect reality.
> > +Most commonly, the counters will read as 0, indicating that they are not
> > +enabled. If proper support is not provided in firmware it's best to disable
> > +CONFIG_ARM64_AMU_EXTN.
> 
> For the sake of one kernel runs everywhere, do we need some other
> mechanism to disable the AMU. e.g kernel parameter to disable amu
> at runtime ?
>

The reason I've not added this is twofold:
 - Even if we add this, it should be in order to disable the use of the
   counters for a certain purpose, in this case  frequency invariance.
   On its own AMU provides the counters but it does not mandate their
   use.
 - I could add something to disable the use of the core and cycle
   counters for frequency invariance at runtime, but I doubt that
   anyone would use it. Logically it makes sense to use the counters
   order to have a more accurate view of the performance that the CPUs
   are actually providing. Therefore, until anyone asks for this, I
   thought it's better to keep it simple and not add extra switches,
   until there is a use for them.

Does it make sense?

P.S. I'll make all the other changes you've suggested in v3. 

Thank you,
Ionela.



> > diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
> > index 5d78a6f5b0ae..a3f1a47b6f1c 100644
> > --- a/Documentation/arm64/booting.rst
> > +++ b/Documentation/arm64/booting.rst
> > @@ -248,6 +248,20 @@ Before jumping into the kernel, the following conditions must be met:
> >       - HCR_EL2.APK (bit 40) must be initialised to 0b1
> >       - HCR_EL2.API (bit 41) must be initialised to 0b1
> > +  For CPUs with Activity Monitors Unit v1 (AMUv1) extension present:
> > +  - If EL3 is present:
> > +    CPTR_EL3.TAM (bit 30) must be initialised to 0b0
> > +    CPTR_EL2.TAM (bit 30) must be initialised to 0b0
> > +    AMCNTENSET0_EL0 must be initialised to 0b1111
> > +    AMCNTENSET1_EL0 must be initialised to a platform specific value
> > +    having 0b1 set for the corresponding bit for each of the auxiliary
> > +    counters present.
> > +  - If the kernel is entered at EL1:
> > +    AMCNTENSET0_EL0 must be initialised to 0b1111
> > +    AMCNTENSET1_EL0 must be initialised to a platform specific value
> > +    having 0b1 set for the corresponding bit for each of the auxiliary
> > +    counters present.
> > +
> >   The requirements described above for CPU mode, caches, MMUs, architected
> >   timers, coherency and system registers apply to all CPUs.  All CPUs must
> >   enter the kernel in the same exception level.
> > diff --git a/Documentation/arm64/cpu-feature-registers.rst b/Documentation/arm64/cpu-feature-registers.rst
> > index b6e44884e3ad..4770ae54032b 100644
> > --- a/Documentation/arm64/cpu-feature-registers.rst
> > +++ b/Documentation/arm64/cpu-feature-registers.rst
> > @@ -150,6 +150,8 @@ infrastructure:
> >        +------------------------------+---------+---------+
> >        | DIT                          | [51-48] |    y    |
> >        +------------------------------+---------+---------+
> > +     | AMU                          | [47-44] |    n    |
> > +     +------------------------------+---------+---------+
> 
> As mentioned above, please drop it.
> 
> 
> Suzuki

  reply	other threads:[~2020-01-30 16:45 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-18 18:26 [PATCH v2 0/6] arm64: ARMv8.4 Activity Monitors support Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 1/6] arm64: add support for the AMU extension v1 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 18:32     ` Ionela Voinescu
2020-01-24 12:00       ` Valentin Schneider
2020-01-28 11:00         ` Ionela Voinescu
2020-01-28 16:34   ` Suzuki Kuruppassery Poulose
2020-01-29 16:42     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 2/6] arm64: trap to EL1 accesses to AMU counters from EL0 Ionela Voinescu
2020-01-23 17:04   ` Valentin Schneider
2020-01-23 17:34     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests Ionela Voinescu
2020-01-27 15:33   ` Valentin Schneider
2020-01-28 15:48     ` Ionela Voinescu
2020-01-28 17:26     ` Suzuki Kuruppassery Poulose
2020-01-28 17:37       ` Valentin Schneider
2020-01-28 17:52         ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension Ionela Voinescu
2020-01-27 16:47   ` Valentin Schneider
2020-01-28 16:53     ` Ionela Voinescu
2020-01-28 18:36       ` Valentin Schneider
2020-01-30 15:04   ` Suzuki Kuruppassery Poulose
2020-01-30 16:45     ` Ionela Voinescu [this message]
2020-01-30 18:26       ` Suzuki K Poulose
2020-01-31  9:54         ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 5/6] TEMP: sched: add interface for counter-based frequency invariance Ionela Voinescu
2020-01-29 19:37   ` Peter Zijlstra
2020-01-30 15:33     ` Ionela Voinescu
2019-12-18 18:26 ` [PATCH v2 6/6] arm64: use activity monitors for " Ionela Voinescu
2020-01-23 11:49   ` Lukasz Luba
2020-01-23 17:07     ` Ionela Voinescu
2020-01-24  1:19       ` Lukasz Luba
2020-01-24 13:12         ` Ionela Voinescu
2020-01-24 15:17           ` Lukasz Luba
2020-01-28 17:36             ` Ionela Voinescu
2020-01-29 17:13   ` Valentin Schneider
2020-01-29 17:52     ` Ionela Voinescu
2020-01-29 23:39     ` Valentin Schneider
2020-01-30 15:49       ` Ionela Voinescu
2020-01-30 16:11         ` Valentin Schneider

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200130164542.GC5208@arm.com \
    --to=ionela.voinescu@arm.com \
    --cc=catalin.marinas@arm.com \
    --cc=corbet@lwn.net \
    --cc=dietmar.eggemann@arm.com \
    --cc=ggherdovich@suse.cz \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mark.rutland@arm.com \
    --cc=maz@kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=sudeep.holla@arm.com \
    --cc=suzuki.poulose@arm.com \
    --cc=vincent.guittot@linaro.org \
    --cc=will@kernel.org \
    --subject='Re: [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).