linux-kernel - Re: [PATCH 11/12] perf, x86: implement AMD IBS event configuration

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Message-ID: <o2ibd4cb8901004210202i1ba91c3dlc775a9f7759ece68@mail.gmail.com>
Date:	Wed, 21 Apr 2010 11:02:42 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Robert Richter <robert.richter@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/12] perf, x86: implement AMD IBS event configuration

On Wed, Apr 21, 2010 at 10:47 AM, Robert Richter <robert.richter@....com> wrote:
> On 20.04.10 18:05:57, Robert Richter wrote:
>> > What is the problem with directly using the period here, rejecting
>> > any value that is off range or with bottom 4 bits set?
>>
>> Yes, I will create an updated version of this patch.
>
> Stephane, do you think having the lower 4 bits set is worth an EINVAL?
> I would rather ignore them since the accuracy is not really necessary
> compared to a range lets say from 100000 cycles? Otherwise this will
> make the setup of ibs much more complicated. The check could be moved
> to userland and generate a waring or so.

Explain why you think it would be more complicated?
If I recall there is already a function to validate the attrs:
amd_pmu_hw_config().
But may be you are talking about userland setup.

Here is one argument why this might be important. Some people like to
know exactly
the sampling period because they use a particular value, like a prime
number. You
chopping off the bottom 4 bits could break this logic silently.
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