[Intel-gfx] [RFC 8/8] drm/i915: Support for retrieving MMIO register values alongwith timestamps through perf

[Gupta, Sourab]

On Wed, 2015-08-05 at 10:03 +0000, Chris Wilson wrote:
> On Wed, Aug 05, 2015 at 11:25:44AM +0530, sourab.gupta at intel.com wrote:
> > From: Sourab Gupta <sourab.gupta at intel.com>
> > 
> > This patch adds support for retrieving MMIO register values alongwith
> > timestamps and forwarding them to userspace through perf.
> > The userspace can request upto 8 MMIO register values to be dumped.
> > The addresses of upto 8 MMIO registers can be passed through perf attr
> > config. The registers are checked against a whitelist before passing them
> > on. The commands to dump the values of these MMIO registers are then
> > inserted into the ring alongwith commands to dump the timestamps.
> 
> The values reported to userspace are deltas across batches right? We
> don't expose the global value to an unprivileged user? It would be nice
> to clarify that in perf_init so that the reviewer is aware that
> the issue of unprivileged information leak is addressed (or at least
> reminded that the register values do not leak!).
> -Chris
> 
Hi Chris,
Two things here:
1) Only root is allowed to call event_init for gen pmu. This restriction
is there in event_init. (The thought behind this restriction being that
we are profiling data across contexts here, so a process wishing to
listen to global activity happening in system across all contexts ought
to have root priviliges). Is this thought process correct? Should we be
supporting non-root users too?

2) Being already a root, do we need to worry about the unauthorized mmio
access while exposing these mmio values through the interface?

In the current patches, the full mmio register value is dumped to be
passed on to userspace (no deltas across batches), provided the register
is there in the whitelist. Does the question of unpriviliged information
leak arise here(the user being root)?

Regards,
Sourab