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From: Andi Kleen <andi@firstfloor.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        vince@deater.net, eranian@google.com,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] perf/x86/intel/pt: Allow disabling branch tracing
References: <87inosaoi1.fsf@ashishki-desk.ger.corp.intel.com>
        <20170206144140.14402-1-alexander.shishkin@linux.intel.com>
Date: Mon, 06 Feb 2017 07:58:25 -0800
In-Reply-To: <20170206144140.14402-1-alexander.shishkin@linux.intel.com>
        (Alexander Shishkin's message of "Mon, 6 Feb 2017 16:41:40 +0200")
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Alexander Shishkin <alexander.shishkin@linux.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