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

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Date:	Tue, 23 Apr 2013 09:34:23 -0500
From:	Jacob Shin <jacob.shin@....com>
To:	Will Deacon <will.deacon@....com>
CC:	Ingo Molnar <mingo@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	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>
Subject: Re: [PATCH V2 1/4] perf: Add hardware breakpoint address mask

On Tue, Apr 23, 2013 at 10:54:37AM +0100, Will Deacon wrote:
> Hi Jacob,
> 
> On Tue, Apr 23, 2013 at 08:57:02AM +0100, Jacob Shin wrote:
> > Some architectures (for us, AMD Family 16h) allow for "don't care" bit
> > mask to further qualify a hardware breakpoint address, in order to
> > trap on range of addresses. Update perf uapi to add bp_addr_mask field.
> 
> arm and arm64 have a similar feature to this, whereby we currently have to
> translate the bp_len field into a mask, which is all the hardware
> understands. Unlike what you describe, our mask indicates the bytes we *are*
> interested in, but I think we could make use of the same functionality that
> you're introducing here.
> 
> There are some funky restrictions on the alignment of the base address, but
> we can detect those and tell userspace where to go if it tries any funny
> stuff.
> 
> Can you see a problem if I simply invert the mask?

Hi,

That's great! No, I don't see a problem at all.

I guess now it can be debated if the mask coming in from userland should
be include or exclude mask. But I think exclude makes syntax easier:

To count writes to [0x1000 ~ 0x1010)

Include mask (my current patchset):

  perf stat -e mem:0x1000/0xf:w a.out

Exclude mask:

  perf stat -e mem:0x1000/0xfff0:w a.out

Thanks!

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