linux-kernel - Re: [PATCH V2 1/4] hw_breakpoint: Add step

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Date:   Tue, 25 Jul 2017 16:14:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Pratyush Anand <panand@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        huawei.libin@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/4] hw_breakpoint: Add step_needed event attribute

On Tue, Jul 25, 2017 at 02:27:38PM +0100, Will Deacon wrote:
> On Fri, Jul 07, 2017 at 05:33:57PM +0530, Pratyush Anand wrote:
> > Architecture like ARM64 currently allows to use default hw breakpoint
> > single step handler only to perf. However, some other users like few
> > systemtap tests or kernel test in
> > samples/hw_breakpoint/data_breakpoint.c can also work with default step
> > handler implementation. At the same time, some other like GDB/ptrace may
> > implement their own step handler.
> > 
> > Therefore, this patch introduces a new perf_event_attr bit field, so
> > that arch specific code(specially on arm64) can make a decision to
> > enable single stepping.
> > 
> > Any architecture which is not using this field will not have any
> > side effect.

> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index b1c0b187acfe..00935808de0d 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -345,7 +345,8 @@ struct perf_event_attr {
> >  				context_switch :  1, /* context switch data */
> >  				write_backward :  1, /* Write ring buffer from end to beginning */
> >  				namespaces     :  1, /* include namespaces data */
> > -				__reserved_1   : 35;
> > +				step_needed    :  1, /* Use arch step handler */
> > +				__reserved_1   : 34;
> 
> This needs documenting properly, as I really have no idea how userspace is
> going to use it sensibley, especially as you silently overwrite it in some
> cases below.

This is not something userspace _can_ use sensibly afaict. Therefore it
should probably not live here.