Re: [PATCH -tip v3 2/5] perf: change perf_event_header.misc to PERF_RECORD_MISC_USER for BTS

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2011-08-11 at 21:06 +0900, Akihiro Nagai wrote:
> Change perf_event_headder.misc to PERF_RECORD_MISC_USER for
> BTS records, because BTS traces both kernel and user spaces
> nevertheless perf specifies to trace only kernel or user space.

Now I'm confused..

If BTS traces both kernel and user, the MISC bit should reflect the
right state per-sample, on x86 that's easy enough to do by the address.


> ---
> 
>  arch/x86/kernel/cpu/perf_event_intel_ds.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> index 1b1ef3a..323f3f0 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -340,6 +340,14 @@ static int intel_pmu_drain_bts_buffer(void)
>  	 */
>  	perf_prepare_sample(&header, &data, event, &regs);
>  
> +	/*
> +	 * Since BTS can not trace kernel and user space separately, set

Uhm, IA32_DEBUGCTL_MSR.BTS_OFF_{OS,USR} seem to suggest it can?!

> +	 * PERF_RECORD_MISC_USER in header.misc to resolve both kernel and
> +	 * user DSOs and symbols.
> +	 */
> +	header.misc &= ~PERF_RECORD_MISC_CPUMODE_MASK;
> +	header.misc |= PERF_RECORD_MISC_USER;


So what's wrong with something like:

	header.misc |= is_kernel_address(at->from) ?
			PERF_RECORD_MISC_KERNEL :
			PERF_RECORD_MISC_USER;

>  	if (perf_output_begin(&handle, event, header.size * (top - at)))
>  		return 1;
>  
>