linux-kernel - Re: [PATCH 04/12] powerpc: Implement hw_breakpoint_arch

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Date:   Fri, 1 Jun 2018 16:46:08 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Joel Fernandes <joel.opensrc@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Chris Zankel <chris@...kel.net>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Rich Felker <dalias@...c.org>, Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH 04/12] powerpc: Implement hw_breakpoint_arch_parse()

On Mon, May 28, 2018 at 09:31:07PM +1000, Michael Ellerman wrote:
> Frederic Weisbecker <frederic@...nel.org> writes:
> 
> > On Thu, May 24, 2018 at 12:01:52PM +1000, Michael Ellerman wrote:
> >> Frederic Weisbecker <frederic@...nel.org> writes:
> >> 
> >> > diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> >> > index 348cac9..fba6527 100644
> >> > --- a/arch/powerpc/kernel/hw_breakpoint.c
> >> > +++ b/arch/powerpc/kernel/hw_breakpoint.c
> >> > @@ -139,30 +139,31 @@ int arch_bp_generic_fields(int type, int *gen_bp_type)
> >> >  /*
> >> >   * Validate the arch-specific HW Breakpoint register settings
> >> >   */
> >> > -int arch_validate_hwbkpt_settings(struct perf_event *bp)
> >> > +int hw_breakpoint_arch_parse(struct perf_event *bp,
> >> > +			     struct perf_event_attr *attr,
> >> > +			     struct arch_hw_breakpoint *hw)
> >> 
> >> I think the semantics here are that we are reading from bp/attr and
> >> writing to hw?
> >> 
> >> If so would some sprinkling of const on the first two parameters help
> >> make that clearer?
> >
> > I seem to remember there was an issue with that due to the various functions
> > we call that need to be converted to take const as well. I thought I would
> > do it in a seperate series but actually it should be no big deal to do it
> > on this one.
> 
> Yeah, that does sometimes snowball out of control.
> 
> > Let me try that and respin.
> 
> Cool. It would be nice to have, but obviously not crucial.

So I managed to constify the perf_event_attr parameter but not the struct perf_event *bp
because the task target is fetched from it on is_compat_bp() on ARM64. I could constify it
all the way up to test_ti_thread_flag() but the thread info can only be retrieved through
a call to task_thread_info() and that's where the qualifier control ends. The const can not
be passed there and we can't afford to constify the function either, I fear, as it is used
everywhere for any purpose, including thread_info modifications.