Re: [PATCH 2/2] perf/x86/intel/pt: Allow disabling branch tracing

Ingo Molnar
2017-02-01T10:17:11Z

* Alexander Shishkin <alexander.shishkin@linux.intel.com> wrote:

> Now that Intel PT supports more types of trace content than just branch
> tracing, it may be useful to allow the user to disable branch tracing
> when it is not needed.
>
> The special case is BDW, where not setting BranchEn is not supported.
>
> This patch adds 'no_branch' event format string to PT events, which
> will disable setting BranchEn bit in the hardware trace configuration.

> + /* trying to unset BRANCH_EN where it is not supported */

Please capitalize comments consistently and use the typical tense. This one should
be something like:

/* Try to unset BRANCH_EN where it is not supported: */

>
> reg = pt_config_filters(event);
> - reg |= RTIT_CTL_TOPA | RTIT_CTL_BRANCH_EN | RTIT_CTL_TRACEEN;
> + reg |= RTIT_CTL_TOPA | RTIT_CTL_TRACEEN;
> +
> + /*
> + * Previously, we had BRANCH_EN on by default, but now that PT has
> + * grown features outside of branch tracing, it is useful to allow
> + * the user to disable it. So, to keep compatibility, setting
> + * BRANCH_EN bit in the event config (no_branch=1) will have the
> + * reverse effect and *not* set BRANCH_EN in the hardware
> + * configuration.
> + */
> + if (!(event->attr.config & RTIT_CTL_BRANCH_EN))
> + reg |= RTIT_CTL_BRANCH_EN;
> + else
> + event->attr.config &= ~RTIT_CTL_BRANCH_EN;

So I really hate this ABI hack - it's these unclean approaches how ABIs degrade
over time, by death of a thousand cuts...

Any reason why we couldn't add a separate pt_feature_branch_disable and
pt_feature_trace_disable bits and interpret them in a straightforward way, or
something like that?

( This means two more bits, but that's our punishment for not anticipating
extensions to the hardware feature. )

Also, rename "RTIT_CTL_BRANCH_EN" to "RTIT_CTL_PT_EN" (but without changing the
ABI), to more clearly express what that bit realy does.

I.e. we'd have a hierarchy of flags:

- the old RTIT_CTL_BRANCH_EN bit (now RTIT_CTL_PT_EN) enables all of PT,
with all features

- individual feature disabling bits, which default to 0 (i.e. the feature
is enabled) in the attr structure control the finegrained
enabling/disabling of PT features. Currently there are two bits:
pt_feature_branch_disable and pt_feature_branch_enable. More are added
in the future if PT grows even more features.

or so?

Thanks,

Ingo