From: Mark Rutland <mark.rutland@arm.com>
To: Raphael Gault <raphael.gault@arm.com>
Cc: raph.gault+kdev@gmail.com, peterz@infradead.org,
catalin.marinas@arm.com, will.deacon@arm.com,
linux-kernel@vger.kernel.org, acme@kernel.org, mingo@redhat.com,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/5] arm64: cpufeature: Add feature to detect heterogeneous systems
Date: Tue, 20 Aug 2019 16:49:55 +0100 [thread overview]
Message-ID: <20190820154955.GB43412@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190820152316.GA38082@lakrids.cambridge.arm.com>
On Tue, Aug 20, 2019 at 04:23:17PM +0100, Mark Rutland wrote:
> Hi Raphael,
>
> On Fri, Aug 16, 2019 at 01:59:31PM +0100, Raphael Gault wrote:
> > This feature is required in order to enable PMU counters direct
> > access from userspace only when the system is homogeneous.
> > This feature checks the model of each CPU brought online and compares it
> > to the boot CPU. If it differs then it is heterogeneous.
>
> It would be worth noting that this patch prevents heterogeneous CPUs
> being brought online late if the system was uniform at boot time.
Looking again, I think I'd misunderstood how
ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU was dealt with, but we do have a
problem in this area.
[...]
>
> > + .capability = ARM64_HAS_HETEROGENEOUS_PMU,
> > + .type = ARM64_CPUCAP_SCOPE_LOCAL_CPU | ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU,
> > + .matches = has_heterogeneous_pmu,
> > + },
I had a quick chat with Will, and we concluded that we must permit late
onlining of heterogeneous CPUs here as people are likely to rely on
late CPU onlining on some heterogeneous systems.
I think the above permits that, but that also means that we need some
support code to fail gracefully in that case (e.g. without sending
a SIGILL to unaware userspace code).
That means that we'll need the counter emulation code that you had in
previous versions of this patch (e.g. to handle potential UNDEFs when a
new CPU has fewer counters than the previously online CPUs).
Further, I think the context switch (and event index) code needs to take
this cap into account, and disable direct access once the system becomes
heterogeneous.
Thanks,
Mark.
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-08-20 15:50 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-16 12:59 [PATCH v3 0/5] arm64: Enable access to pmu registers by user-space Raphael Gault
2019-08-16 12:59 ` [PATCH v3 1/5] perf: arm64: Add test to check userspace access to hardware counters Raphael Gault
2019-08-16 12:59 ` [PATCH v3 2/5] arm64: cpufeature: Add feature to detect heterogeneous systems Raphael Gault
2019-08-20 15:23 ` Mark Rutland
2019-08-20 15:49 ` Mark Rutland [this message]
2019-08-20 15:55 ` Raphael Gault
2019-08-20 16:03 ` Mark Rutland
2019-08-16 12:59 ` [PATCH v3 3/5] arm64: pmu: Add function implementation to update event index in userpage Raphael Gault
2019-08-20 15:34 ` Mark Rutland
2019-08-16 12:59 ` [PATCH v3 4/5] arm64: perf: Enable pmu counter direct access for perf event on armv8 Raphael Gault
2019-08-18 12:37 ` kbuild test robot
2019-08-19 7:59 ` Raphael Gault
2019-08-20 6:49 ` [kbuild-all] " Philip Li
2019-08-16 12:59 ` [PATCH v3 5/5] Documentation: arm64: Document PMU counters access from userspace Raphael Gault
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=20190820154955.GB43412@lakrids.cambridge.arm.com \
--to=mark.rutland@arm.com \
--cc=acme@kernel.org \
--cc=catalin.marinas@arm.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=raph.gault+kdev@gmail.com \
--cc=raphael.gault@arm.com \
--cc=will.deacon@arm.com \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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).