linux-kernel - Re: [PATCH 1/3] arm/pmu: Reject groups spanning multiple hardware PMUs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 10 Mar 2015 13:53:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Suzuki Poulose <Suzuki.Poulose@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"acme@...nel.org" <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Punit Agrawal <Punit.Agrawal@....com>,
	Pawel Moll <Pawel.Moll@....com>
Subject: Re: [PATCH 1/3] arm/pmu: Reject groups spanning multiple hardware
 PMUs

On Tue, Mar 10, 2015 at 12:05:21PM +0000, Mark Rutland wrote:
> On Tue, Mar 10, 2015 at 11:27:23AM +0000, Peter Zijlstra wrote:
> > On Mon, Mar 09, 2015 at 12:46:30PM +0000, Suzuki K. Poulose wrote:
> > > From: "Suzuki K. Poulose" <suzuki.poulose@....com>
> > > 
> > > Don't allow grouping hardware events from different PMUs
> > >  (eg. CCI + CPU).
> > 
> > Uhm, how does this work? If we have multiple hardware PMUs we'll stop
> > scheduling events after the first failed event schedule. This can leave
> > one of the PMUs severely under utilized.
> 
> The problem is here group validation at pmu::event_init() time, not
> scheduling.

Maybe make that a little more explicit.

> We don't allow grouping across disparate HW PMUs because we can't
> provide group semantics anyway. Scheduling is not a problem in this case
> (unlike the big.LITTLE case I have a patch for [1]).

Right, I remember that; I was wondering if this was related.

> We have a CPU PMU and an "uncore" CCI PMU. You can't create task-bound
> events for the CCI, but you can create CPU-bound events for the CCI on
> the nominal CPU the CCI is monitored from.

Indeed, ok.

> The context check you added in c3c87e770458aa00 "perf: Tighten (and fix)
> the grouping condition" implicitly rejects groups that have CPU and CCI
> events (each event::ctx will be the relevant pmu::pmu_cpu_context and
> will differ), and this is sane -- you can't provide group semantics
> across disparate HW PMUs.

Agreed.

> Unfortunately that happens after we've done the
> event->pmu->event_init(event) dance on each event, and in our event_init
> function we try to verify the group is sane. In our verification we
> ignore SW events, but assume that all !SW events are for the CPU PMU.
> If you add a CPU event to a CCI group, that's not the case, and we use
> container_of on an unsuitable object, derefence garbage, invoke the
> eschaton and so on.

Indeed, on x86 we explicitly ignore everything not an x86_pmu event.

> It would be nicer if we could prevent this in the core so we're not
> reliant on every PMU driver doing the same verification. My initial
> thought was that seemed like unnecessary duplication of the ctx checking
> above, but if we're going to end up shoving it into several drivers
> anyway perhaps it's the lesser evil.

Again, agreed, that would be better and less error prone. But I'm not
entirely sure how to go about doing it :/ I'll have to go think about
that; and conferences are not the best place for that.

Suggestions on that are welcome of course ;)
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/