linux-kernel - Re: [PATCH 2/9] perf: Remove pointless union that wraps the hw breakpoint fields

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Message-ID: <20091207014645.GD4966@nowhere>
Date:	Mon, 7 Dec 2009 02:46:47 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"K. Prasad" <prasad@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 2/9] perf: Remove pointless union that wraps the hw
	breakpoint fields

On Sun, Dec 06, 2009 at 11:38:55AM +0100, Peter Zijlstra wrote:
> On Sun, 2009-12-06 at 08:34 +0100, Frederic Weisbecker wrote:
> > It stands to anonymize a structure, but structures can already
> > anonymize by themselves.
> > 
> > Reported-by: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Cc: "K. Prasad" <prasad@...ux.vnet.ibm.com>
> > ---
> >  include/linux/perf_event.h |   14 ++++++--------
> >  1 files changed, 6 insertions(+), 8 deletions(-)
> > 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index a61e4de..53230e9 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -215,14 +215,12 @@ struct perf_event_attr {
> >  		__u32		wakeup_watermark; /* bytes before wakeup   */
> >  	};
> >  
> > -	union {
> > -		struct { /* Hardware breakpoint info */
> > -			__u64		bp_addr;
> > -			__u32		bp_type;
> > -			__u32		bp_len;
> > -			__u64		__bp_reserved_1;
> > -			__u64		__bp_reserved_2;
> > -		};
> > +	struct { /* Hardware breakpoint info */
> > +		__u64		bp_addr;
> > +		__u32		bp_type;
> > +		__u32		bp_len;
> > +		__u64		__bp_reserved_1;
> > +		__u64		__bp_reserved_2;
> >  	};
> >  
> >  	__u32			__reserved_2;
> 
> So I'm a bit puzzled by the need for
>  - that structure to begin with



It has no practical use. It's just a logical separation
that makes it easier to review.

I won't mind much if you prefer to remove it.


>  - specialized __bp reserves


Because we'll probably have further new needs in the future
in the breakpoint fields. But well, these can map to the
current reserved fields already.



> Furthermore, you still got the packing wrong, leading to different
> structure layout on 32 and 64 bit platforms,..
> 
> How about?
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 89098e3..5595154 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -215,17 +215,12 @@ struct perf_event_attr {
>  		__u32		wakeup_watermark; /* bytes before wakeup   */
>  	};
>  
> -	struct { /* Hardware breakpoint info */
> -		__u64		bp_addr;
> -		__u32		bp_type;
> -		__u32		bp_len;
> -		__u64		__bp_reserved_1;
> -		__u64		__bp_reserved_2;
> -	};
> -
>  	__u32			__reserved_2;
>  
> -	__u64			__reserved_3;
> +	/* Hardware breakpoint info */
> +	__u64		bp_addr;
> +	__u32		bp_type;
> +	__u32		bp_len;
>  };



Right this fixes the packing layout, but what if
we need other fields for the breakpoints in the future?

Thanks.

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