linux-kernel - Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN

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Message-ID: <20131204135743.GB7251@redhat.com>
Date:	Wed, 4 Dec 2013 14:57:43 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	jacob.w.shin@...il.com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Melo <acme@...stprotocols.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Sherry Hurwitz <sherry.hurwitz@....com>
Subject: Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len >
	HW_BREAKPOINT_LEN_8

On 12/03, Frederic Weisbecker wrote:
>
> 2013/11/11 Oleg Nesterov <oleg@...hat.com>:
> > On 11/11, Frederic Weisbecker wrote:
> >>
> >> On Sat, Nov 09, 2013 at 04:54:28PM +0100, Oleg Nesterov wrote:
> >> >
> >> > Up to you and Suravee, but can't we cleanup this later?
> >> >
> >> > This series was updated many times to address a lot of (sometimes
> >> > contradictory) complaints.
> >>
> >> Sure. But I'm confident that we can solve the conflicting mask / len issue easily beside.
> >> I mean, I don't feel confident with merging things as is, otoh it should be easy to fix up.
> >
> > I do not really understand where do you see the conflict...
> >
> > I can be easily wrong, but afaics currently mask / len issue is simply
> > the implementation detail.
>
> I think it's like we have an object that has a length, and to create
> this object we pass both kilometers and miles. Ok it's a bit different
> here because a mask can apply on top of a len. But here it's used to
> define essentially the same thing (ie: a range of address)

Yes. perf/etc uses length, the current imlementation uses ->mask to
actually set the range.

> > Actually, mask is more powerfull. And initial versions of this patches
> > (iirc) tried to use mask as an argument which comes from the userspace
> > (tools/perf, perf_event_attr, etc). But one of reviewers nacked this
> > interfacer, so we still use len.
>
> Well, we can still reconsider it if needed but to me it seems that
> mask is only interesting if we may deal with non contiguous range of
> addresses.

And this is what this mask can actually do. Just there is no way (currently)
to pass the mask from userpace.

> >> Right but what if we want breakpoints having a size below 8? Like break on instructions
> >> from 0x1000 to 0x1008 ?
> >>
> >> Or should we ignore range instruction breakpoints when len < 8?
> >
> > In this case the new code has no effect (iirc), we simply use
> > X86_BREAKPOINT_LEN_* and "tell the hardware about extended range/mask"
> > code is never called. IIRC, currently we simply check bp_mask != 0
> > to distinguish.
>
> I'm not sure I understand correctly. Do you mean that range below 8
> don't rely on extended breakpoint range?

IIRC - yes.

> Ideally it would be nice if we drop bp_mask and use extended ranges
> only when len > 8. How does that sound?

Again, iirc, this is what the code does. except (in essence) it checks
mask != 0 instead of len > 8.

And yes, we can probably drop bp_mask (unless we are going to support
the contiguous ranges), just I think we can do this later.

Oleg.

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