To add an example: the whole OFFCORE_EVENTS issue on Nehalem processors with perf.
Currently it is impossible for external tools to detect if this feature is enabled or not. If you try to use it, sometimes it will return 0, sometimes not (depending which generalized event you've used due to a buggy leak of MSR state). This bug has been there since 2.6.39. What should happen is an error code returned.
If RAW OFFCORE_EVENT support is ever added, then there is no possible way for an external tool to detect this properly, as correct behavior is indistinguishable from the current buggy disabling.
Does the perf tool care? No. Why? Because when it gets support it will happen at the same time that the kernel is updated, so as long as kernel/perf are updated at the same time you'll never notice. perf never has to worry about backward compatability, which is a bit of an unfair advantage.
Copyright © 2017, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds