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

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Message-ID: <20170206171921.GG26852@two.firstfloor.org>
Date:   Mon, 6 Feb 2017 09:19:21 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        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

On Mon, Feb 06, 2017 at 06:05:29PM +0200, Alexander Shishkin wrote:
> Andi Kleen <andi@...stfloor.org> writes:
> 
> > 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/
> 
> It's /passthrough=1,branch=0/ or simply /passthrough=1/.

Ok, but still you have to list exactly to which flags passthrough
applies to, and it will only ever be branchen.

So basically you turned nobranch=1 into two more difficult to
explain flags without any future advantage.

That is why nobranch=1 is better. It is far easier to explain
and logical to the user.

-Andi