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

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Message-ID: <20130424094853.GB21850@mudshark.cambridge.arm.com>
Date:	Wed, 24 Apr 2013 10:48:53 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jacob Shin <jacob.shin@....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" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] perf: Add hardware breakpoint address mask

On Tue, Apr 23, 2013 at 04:18:46PM +0100, Jacob Shin wrote:
> On Tue, Apr 23, 2013 at 04:02:40PM +0100, Will Deacon wrote:
> > On Tue, Apr 23, 2013 at 03:40:57PM +0100, Jacob Shin wrote:
> > > >   perf stat -e mem:0x1000/0xf:w a.out
> > 
> > Are you saying that this command would count any write to:
> > 
> > 	0x1000
> > 	0x1001
> > 	...
> > 	0x100e
> > 	0x100f
> > 
> > ?
> > 
> > If so, that differs from the ARM debug architecture in that the mask is called
> > `byte-address-select', so a mask of 0b1001 would count accesses at +0 bytes
> > and +3 bytes from the base address. Is that possible to describe with your
> > masking scheme and a single watchpoint?
> > 
> > A mask of 0xf, would count +0, +1, +2 and +3 (essentially bp_len == 4).
> > 
> > Unfortunately, that means I can't just invert the mask like I originally
> > thought.
> 
> Ah, .. that is different .
> 
> Our hardware matches on the breakpoint if:
> 
>   (physical_address & ~bp_addr_mask) == (bp_addr & ~bp_addr_mask)
> 
> In other words, the mask says which of the bp_addr bits hardware should
> ignore when matching.
> 
> .. it would be great if we can come up with userland interface that works
> for both archs. I'm coming up empty at the moment ..

After a bit of thought, I *think* the ARM mechanism is more expressive, and
you could describe your masking in terms of byte-address-select. The
downside is that we'd end up with much larger masks, which could be argued
as counter-intuitive from a user's point of view.

Will
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