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

[Akihiro Nagai]

(2011/08/11 21:18), Peter Zijlstra wrote:
> 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.
Yes.
However, PERF_RECORD_MISC_KERNEL can be specified only when
both from_addr and to_addr are kernel-space. Since current perf always enables
IA32_DEBUGCTL_MSR.BTS_OFF_OS flag when it uses BTS, such BTS records are not
output. So, it's enough to specify only PERF_RECORD_MISC_USER.

> > ---
> >
> >   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?!
Current perf always enables IA32_DEBUGCTL_MSR.BTS_OFF_OS when traces with BTS.
However, BTS records branches which jump from kernel(irq_return) to user, because
the msr is to stop tracing in Ring0. It's different with kernel-space in the
strict sense.

> > +	 * 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;
It looks good.
However, current perf doesn't output "kernel to kernel" BTS records.
So, it is unnecessary yet.

Thank you.

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

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