linux-kernel - Re: [PATCH V2 1/4] perf: Add hardware breakpoint address mask

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Date:	Thu, 25 Apr 2013 18:19:11 -0500
From:	Jacob Shin <jacob.shin@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Oleg Nesterov <oleg@...hat.com>, Will Deacon <will.deacon@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Jiri Olsa <jolsa@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] perf: Add hardware breakpoint address mask

On Thu, Apr 25, 2013 at 10:17:35AM -0700, H. Peter Anvin wrote:
> On 04/25/2013 10:06 AM, Oleg Nesterov wrote:
> >>
> >> The downside is that in userland perf tool we need differing documentation
> >> on what the mask syntax means for each architecture.
> > 
> > Personally I think this is acceptable.
> > 
> > But I am new to this code, so...
> > 
> 
> That would seem really, really awkward.  Yes, perf has a bunch of
> low-level stuff, but it would seem highly undesirable to force the user
> to deal with something like that.
> 
> It would be good to have a user-friendly syntax that covers most of what
> users may want to do and perhaps a longer form that can express
> everything including ARM's byte selects; if the system can't honor the
> request it should return an error.

Okay,

If arch specific masks are a no go, then I think I'm convinced that
Oleg's idea of using bp_len is the right thing to do. Right now perf
userland tool hard codes bp_len to 4, so I need to modify it to allow
user to override the length if desired.

Oleg, Frederic, et al.

Which syntax do you prefer?

If we want to set bp_len to 16:

  $ perf stat -e mem:0x1000:rw:16

Or

  $ perf stat -e mem:0x1000:16

Or

  $ perf stat -e mem:0x1000/16

If no bp_len value is specified, it will still default to 4 as it did
before.

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