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

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Message-ID: <20130425173356.GA2194@redhat.com>
Date:	Thu, 25 Apr 2013 19:33:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jacob Shin <jacob.shin@....com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.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,
	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 04/25, Jacob Shin wrote:
>
> On Thu, Apr 25, 2013 at 05:10:35PM +0200, Oleg Nesterov wrote:
> > On 04/25, Frederic Weisbecker wrote:
> > >
> > > Do we need len and mask to work at the same time? I can't think of a
> > > situation when len and mask mix up together in a useful way to define
> > > a range.
>
> Okay, we can make it:
>
> union {
>       __u64 bp_len;
>       __u64 bp_addr_mask;
>       __config2;
> };
>
> And in x86, bp_len != HW_BREAKPOINT_LEN_1,2,4,8 will be interpreted as
> bp_addr_mask.

I think this can work too. And this needs almost the same changes as
extending ->bp_len.

> > Well. Another option is to extend bp_len. Fortunately HW_BREAKPOINT_LEN_*
> > match the length, so we can simply allow any 2^n length and amd.c can
> > translate it into the mask.
>
> Okay, this is nice because we can just ride on top of what already exits,
> but ...

Yes, yes, I agree with your "but". As I said from the very beginning
I am not sure about this idea.

> addr of 0x1000 and mask of 0xf0 will count accesses to:
>
> 0x1000, 0x1010, 0x1020, .. 0x10e0, 0x10f0
>
> Maybe there is some big blob of data and user wants to see how many times
> 16 byte aligned addresses get hit. This might be not as common, but it is
> plausible no?

I'd say this is certainly uncommon ;)

But in any case we should not limit a user, so I agree.

Oleg.

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