From: rusty@rustcorp•com.au (Rusty Russell)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH] ARM: test for PMU feature on v7 (v2 with typo fix)
Date: Sat, 24 Mar 2012 09:47:07 +1030 [thread overview]
Message-ID: <87fwcy9abw.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20120323095314.GA13622@mudshark.cambridge.arm.com>
On Fri, 23 Mar 2012 09:53:14 +0000, Will Deacon <will.deacon@arm•com> wrote:
> Hi Rusty,
>
> On Fri, Mar 23, 2012 at 12:34:57AM +0000, Rusty Russell wrote:
> > The v7 ARM (B4.1.82) specifies that bits 27-24 of the ID_DFR0 reg show
> > what performance monitoring (if any) is available. We should test this
> > before assuming (useful for inside virtualized environments, for example).
>
> Ok, I can see why you don't want to emulate a PMU but I don't agree with the
> patch :)
...
> > static struct arm_pmu *__init armv7_a8_pmu_init(void)
> > {
> > + if (!armv7_pmu_avail())
> > + return NULL;
>
> This code is only executed if we know for sure that we are running on a
> Cortex-A8. Cortex-A8 implementations have a PMU, so I don't like having the
> probe here in case we are running on a virtualised CPU that doesn't quite
> match the hardware.
OK, I see that argument, but I don't like it. eg. the Cortex A-15 is
defined to have a PMUv2 with 6 counters, and below you're arguing we
should implement zero of them.
I had assumed that as we trend towards a common cross-platform kernel,
we should be relying on feature registers where available, not the
model.
> Would you be able to advertise 0 event counters instead and treat the PMU
> largely as RAZ/WI? The only tricky bit I can see is the cycle counter, which
> is mandated by the presence of a PMU, however this could also just RAZ
> initially.
Yes, that's the effect of the current emulation hack. Far better not to
lie to the guest, however, than get weird results from profiling.
Cheers,
Rusty.
--
How could I marry someone with more hair than me? http://baldalex.org
next prev parent reply other threads:[~2012-03-23 23:17 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4F590411.9060201@arm.com>
2012-03-23 0:32 ` [PATCH] ARM: test for PMU feature on v7 Rusty Russell
2012-03-23 0:34 ` [PATCH] ARM: test for PMU feature on v7 (v2 with typo fix) Rusty Russell
2012-03-23 9:53 ` Will Deacon
2012-03-23 23:17 ` Rusty Russell [this message]
2012-03-26 9:40 ` Will Deacon
2012-03-26 9:50 ` [Android-virt] " Peter Maydell
2012-03-26 15:15 ` Will Deacon
2012-03-28 6:09 ` Rusty Russell
2012-03-28 14:17 ` Nicolas Pitre
2012-03-30 17:04 ` Will Deacon
2012-03-30 17:40 ` Nicolas Pitre
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