Re: [RFC 2/3] perf/x86: Control RDPMC access from .enable() hook

[Peter Zijlstra]

On Sun, Aug 29, 2021 at 11:05:55PM -0400, Vince Weaver wrote:

> as the author of those perf_event tests for rdpmc, I have to say if ARM 
> comes up with a cleaner implementation I'd be glad to have x86 transition 
> to something better.
> 
> The rdpmc code is a huge mess and has all kinds of corner cases.  I'm not 
> sure anyone besides the PAPI library tries to use it, and while it's a 
> nice performance improvement to use rdpmc it is really hard to get things 
> working right.
> 
> As a PAPI developer we actually have run into the issue where the CPU 
> switches and we were reporting the wrong results.  Also if I recall (it's 
> been a while) we were having issues where the setup lets you attach to a 
> process on another CPU for monitoring using the rdpmc interface and it 
> returns results even though I think that will rarely ever work in 
> practice.

There's just not much we can do to validate the usage, fundamentally at
RDPMC time we're not running any kernel code, so we can't validate the
conditions under which we're called.

I suppose one way would be to create a mode where RDPMC is disabled but
emulated -- which completely voids the reason for using RDPMC in the
first place (performance), but would allow us to validate the usage.

Fundamentally, we must call RDPMC only for events that are currently
actuve on *this* CPU. Currently we rely on userspace to DTRT and if it
doesn't we have no way of knowing and it gets to keep the pieces.