linux-kernel - Re: [RFC Patch 1/9] Introducing generic hardware breakpoint handler interfaces

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Date:	Fri, 17 Oct 2008 17:34:09 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	prasad@...ux.vnet.ibm.com
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	akpm@...ux-foundation.org, mingo@...e.hu,
	jason.wessel@...driver.com, avi@...ranet.com,
	richardj_moore@...ibm.com
Subject: Re: [RFC Patch 1/9] Introducing generic hardware breakpoint
	handler interfaces

> I agree with you that single-stepping and enablement of a post_handler
> for instruction breakpoints in x86 provides a 'feature' over what the
> underlying processor provides. There may be corner-case issues to it
> (and I'd be glad to hear if somebody has discovered any).

I'm pretty well positive I can come up with several.  But my point is that
the whole subject is hairy and that I want to separate the concerns
properly and deal with all that later, rather than clutter up the initial
review and integration of hw_breakpoint with all that.

> However, I'm afraid if the addition of such a feature in a layer above
> the hw_breakpoint can be done entirely without modifying code that is
> much closer to the hardware - say do_debug().

I didn't say that sorting out single-step wouldn't involve changes to
do_debug.  It probably will.  But that is still quite separate from
hw_breakpoint and I don't agree at all that such later work will have to
(or want to) be rolled into the hw_breakpoint API itself.

> Yet, given that there's sufficient interest to see hw_breakpoint layer
> as one that provides pure wrappers around processor functionality i.e.
> devoid of the overloading of single-stepping function, I will submit my
> next iteration of patches with a single pointer called triggered() as a
> callback function.

Great.

> It's my understanding that Data Breakpoints, set by DABR fire the Data
> Storage Interrupt in PPC only after the data access operation is done
> (except for store operations on atomic writes). Exceptions resulting due
> to a match with IABR (for instructions) is trigger-before.

Ok, I wasn't really trying to be right about PPC at the moment.  The point
is just to expose the true behavior of the hardware in a coherent way.
When working on each arch port, one of course has to be careful to know the
correct details.

> Are you suggesting that the post_handler_supported() in i386/x86_64 be
> modified to return, say UNSUPPORTED_DUE_TO_RF instead of just
> UNSUPPORTED to indicate that post_handler() may be supported if
> single-stepping is used to step-over the instructions rather than use
> RF bit?

I wasn't proposing the exact API details.  I think the API that makes sense
to describe the true hardware functionality uses somewhat different terms,
so "post_handler_supported" doesn't especially make sense.  What I think
makes sense is to say a given hw_breakpoint type "triggers before" or
"triggers after" (since there really is only one event to possibly have a
handler, not two).  The further possibilities of what kind of "triggers
before" it is are that the handler can request completing the instruction
without a re-trigger, or it can't (and just has to disable the breakpoint).
Probably a couple of Boolean-valued inlines covers it most clearly.

Thanks,
Roland
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