Re: [tip:perf/core] perf: Fix broken union in ' structperf_event_mmap_page'

[Vince Weaver]

On Wed, 18 Sep 2013, Peter Zijlstra wrote:

> On Wed, Sep 18, 2013 at 10:19:32AM -0400, Vince Weaver wrote:
> > Can you point to any code that is fixed by the commit?
> 
> I have some, but I don't think a lot of people use it.
> 
> Would you be ok with something like the below? It should preserve
> functionality for code that only cares about cap_usr_rdpmc (PAPI).
> 
> Stephane, does libpfm use any of these?
> 
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -380,8 +380,8 @@ struct perf_event_mmap_page {
>  	union {
>  		__u64	capabilities;
>  		struct {
> -			__u64	cap_usr_time		: 1,
> -				cap_usr_rdpmc		: 1,
> +			__u64	cap_usr_rdpmc		: 1,
> +				cap_usr_time		: 1,
>  				cap_usr_time_zero	: 1,
>  				cap_____res		: 61;
>  		};
> 

It would be nice if there was some way to detect this change; I liked the 
idea of a "cap_usr_fixed" bit.

Even with your change you can't have code that can reliably detect both 
cap_usr_time and cap_usr_rdpmc unless you can guarantee that both 
perf_event.h and the kernel are 3.12 or newer, and it gets more 
complicated if distros backport this patch.

Tools like PAPI often carry around their own copy of perf_event.h and 
people like to move around binaries between machines with different kernel 
versions so things get complicated quickly.

Vince
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