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

[Vince Weaver]

On Mon, 30 Aug 2021, Peter Zijlstra wrote:

> 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.

yes, though it would be nice for cases where things will never work (such 
as process-attach?  I think even if pinned to the same CPU that won't 
work?) Maybe somehow the mmap page could be set in a way to indicate we 
should fall back to the syscall.  Maybe set pc->index to an invalid value 
so we can use the existing syscall fallback code.

We could force every userspace program to know allthe unsupoorted cases 
but it seems like it could be easier and less failure-prone to centralize 
this in the kernel.

I was looking into maybe creating a patch for this but the magic perf 
mmap page implementation is complex enough that I'm not sure I'm qualified 
to mess with it.

Vince