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

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