Re: [Intel-gfx] [RFC] perf: Allow fine-grained PMU access control

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

Hi,

On 19/01/2018 17:10, Tvrtko Ursulin wrote:
> 
> Hi,
> 
> On 19/01/2018 16:45, Peter Zijlstra wrote:
>> On Thu, Jan 18, 2018 at 06:40:07PM +0000, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>
>>> For situations where sysadmins might want to allow different level of
>>> of access control for different PMUs, we start creating per-PMU
>>> perf_event_paranoid controls in sysfs.
>>
>> You've completely and utterly failed to explain why.
> 
> On an abstract level, if there is a desire to decrease the security knob 
> on one particular PMU provider, it is better to be able to do it just 
> for the one, rather for the whole system.
> 
> On a more concrete level, we have customers who want to look at certain 
> i915 metrics, most probably engine utilization or queue depth, in order 
> to make load-balancing decisions. (The two would be roughly analogous to 
> CPU usage and load.)
> 
> This data needs to be available to their userspaces dynamically and 
> would be used to pick a best GPU engine (mostly analogous to a CPU core) 
> to run a particular packet of work.
> 
> It would be impossible to run their product as root, and while one 
> option could be to write a proxy daemon which would allow unprivileged 
> queries, it is also a significant complication which introduces a time 
> shift problem on the PMU data as well.
> 
> So my thinking was that a per-PMU paranoid control should not be a 
> problematic concept in general. And my gut feeling anyway was that not 
> all PMU providers are the same class data, security wise, which was 
> another reason I thought per-PMU controls would be fine.
> 
> There is one more way of thinking about it, and that is that the access 
> control could even be extended to be per-event, and not just per-PMU. 
> That would allow registered PMUs to let the core know which counters are 
> potentially security sensitive, and which are not.
> 
> I've sent another RFC along those lines some time ago, but afterwards 
> I've changed my mind and thought the approach from this patch should be 
> less controversial since it retains all control fully in the perf core 
> and in the hands of sysadmins.

Any thoughts on this one? Is the approach acceptable?

Regards,

Tvrtko