* Michael Ellerman <mich...@ellerman.id.au> wrote:
> On Tue, Jul 09, 2013 at 10:14:34AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 08, 2013 at 10:24:34PM -0400, Vince Weaver wrote:
> > >
> > > So something like they have on ARM?
> > >
> > > vince@pandaboard:/sys/bus/event_source/devices$ ls -l
> > > lrwxrwxrwx 1 root root 0 Jul 8 21:57 ARMv7 Cortex-A9 ->
> > > ../../../devices/ARMv7 Cortex-A9
> > > lrwxrwxrwx 1 root root 0 Jul 8 21:57 breakpoint ->
> > > ../../../devices/breakpoint
> > > lrwxrwxrwx 1 root root 0 Jul 8 21:57 software ->
> > > ../../../devices/software
> > > lrwxrwxrwx 1 root root 0 Jul 8 21:57 tracepoint ->
> > > ../../../devices/tracepoint
> >
> > Right so what I remember of the ARM case is that their /proc/cpuinfo isn't
> > sufficient to identify their PMU. And they don't have a cpuid like
> > instruction
> > at all.
> >
> > > > For the cpu you can obviously just detect what processor you're on with
> > > > cpuid or whatever, but it's a bit of a hack. And that really doesn't
> > > > work for non-cpu PMUs.
> > >
> > > why is it a hack to use cpuid?
> >
> > I agree, for x86 cpuid is perfectly fine, as would /proc/cpuinfo be, I
> > suspect
> > that just the model number is sufficient in most cases, even for uncore
> > stuff.
>
> What about things on PCI? Other strange buses?
>
> As long as everything's in /sys then it should be _possible_ for
> userspace to work out what's what, but it's going to end up with a bunch
> of detection logic and heuristics in the library.
>
> At which point you've just rewritten libpfm4.
Exactly - PMUs enumerated in /sys should be self-identifying, it's a
hardware topology after all ...
Anytime userspace is forced to look into /proc, or into weird places in
/sys it's a FAIL really.
perf ABIs want to be self-identifying and self-sufficient, anytime
userspace is forced to look elsewhere it adds another source of fragility.
And duplication with something that is 'already in /proc' is not a problem
_at all_, these are computers that provide us different views into the
same physical reality with dozens of different abstractions, so
duplication of information is natural and _good_.
Thanks,
Ingo
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