On Tue, Sep 02, 2014 at 08:39:56AM +0300, Adrian Hunter wrote: SNIP
> >>> -e intel_pt/tsc=1,noretcomp=0/ > >>> > >>> I guess I wanted to hear more elaboration why is this better > >>> than the current way we have by defining an alias, like: > >>> > >>> krava alias: "tsc=1,noretcomp=0" > >>> > >>> -e intel_pt/krava/ > >>> > >>> which gives the same result > >> > >> The default value must be provided by perf tools not the kernel e.g. > >> an older version of perf tools will not be aware of new hardware > >> features that the kernel may know about. If the kernel enables > >> new features by default then the tool may fail. Thus the default > >> value has to be under the control of the tools not the kernel, so > >> a sysfs alias will not work. > >> > > > > I dont follow, could u provide some example? > > Imagine that the 'tsc' term did not exist. Intel PT trace data > would not contain TSC packets, and the decoder would not know how to > decode them. Then imagine that a new version of the hardware adds > 'tsc'. It is such a useful feature that we want it by default, but > older versions of the tools don't know how to decode it, so the > kernel cannot turn it on by default. > > > > > why the tool needs to understand.. it will just take the alias > > and apply/use it? The alias follows the format logic, which tells > > how to apply values on perf_event_attr.. and the kernel knows best > > what's supported in its version.. > > It is similar to why the kernel does not select mmap2 by default. The > kernel doesn't know whether the tool supports it. > ook, could you please add some kind of this explanation to the changelog? thanks, jirka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
