linux-kernel - Re: [PATCH] perf/x86/intel/pt: Allow disabling branch tracing

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Message-ID: <87efzbjopq.fsf@firstfloor.org>
Date:   Mon, 06 Feb 2017 07:58:25 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] perf/x86/intel/pt: Allow disabling branch tracing

Alexander Shishkin <alexander.shishkin@...ux.intel.com> writes:

> Now that Intel PT supports more types of trace content than just branch
> tracing, it may be useful to allow the user to disable branch tracing
> when it is not needed.
>
> The special case is BDW, where not setting BranchEn is not supported.
>
> This is slightly trickier than necessary, because up to this moment
> the driver has been setting BranchEn automatically and the userspace
> assumes as much. Instead of reversing the semantics of BranchEn, we
> introduce a 'passthrough' bit, which will forego the default and allow
> the user to set BranchEn to their heart's content.

cpu/passthrough=1,branchen=1/ seems far uglier/more complicanted to me
than the original cpu/nobranch=1/

Just think how you would explain it to the user in the manpage.

-Andi