linux-kernel - Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap

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Message-ID: <20130919084249.GA14112@gmail.com>
Date:	Thu, 19 Sep 2013 10:42:49 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Vince Weaver <vince@...ter.net>
Cc:	hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, adrian.hunter@...el.com,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Fix broken union in ' struct
 perf_event_mmap_page'


* Vince Weaver <vince@...ter.net> wrote:

> On Tue, 17 Sep 2013, Vince Weaver wrote:
> 
> > 
> > This patch somehow breaks the perf-ABI.
> > 
> > If I take a program that reads "mmap->cap_usr_rdpmc" and compile it
> > against the new header with this change (say from 3.12-rc1)
> > and then run it on an old kernel (say 3.11) then I get "0" for
> > cap_usr_rdpmc.
> > 
> > If I take the same program and recompile against the old (without this 
> > patch) header and run it on 3.11, I get the expected "1" value.
> 
> To follow up, the original case:
> 
>         union {
>                 __u64   capabilities;
>                 __u64   cap_usr_time  : 1,
>                         cap_usr_rdpmc : 1,
>                         cap_____res   : 62;
>         };
> 
>         Then mmap->usr_rdpmc=1; gets assembled as
> 
>         400420:       80 0d 11 05 20 00 02    orb    $0x2,0x200511(%rip)        

Hm, how can it be 0x2? Didn't you mix up the two assemblies accidentally?

> 
> The new version
> 
>        union {
>                 __u64   capabilities;
>                 struct {
>                         __u64   cap_usr_time            : 1,
>                                 cap_usr_rdpmc           : 1,
>                                 cap_usr_time_zero       : 1,
>                                 cap_____res             : 61;
>                 };
>         };
> 
>         mmap->usr_rdpmc=1; gets assembled as
> 
> 
>        400427:       80 0d 02 05 20 00 01    orb    $0x1,0x200502(%rip)        
> 
> note the difference in the or value.

Here the value should be 0x2, as cap_usr_rdpmc move to the second bit.

Right?

	Ingo
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