[PATCH 00/17] Add OA functionality to Xe

[Souza, Jose]

On Tue, 2024-05-21 at 09:10 -0700, Dixit, Ashutosh wrote:
> On Tue, 21 May 2024 07:47:58 -0700, Souza, Jose wrote:
> 
> Hi Jose,
> 
> > > Other ask, can you remove this 'Failed to remove unknown OA config'
> > > debug message from xe_oa_remove_config_ioctl()?
> > 
> > Missed 'Insufficient privileges to remove xe OA config', that need to be
> > removed too from xe_oa_remove_config_ioctl().
> > 
> > > Mesa will be using DRM_XE_PERF_OP_REMOVE_CONFIG with config id set to
> > > UINT64_MAX to detect if Xe KMD supports OA counters and if application
> > > has enough permissions to use it.  So it causes dmesg to be flooded
> > > with 'xe 0000:00:02.0: [drm:xe_oa_remove_config_ioctl [xe]] Failed to
> > > remove unknown OA config' messages when running tests suites.
> > > 
> > > Or do you have other suggestion of uAPI that I can use.
> 
> OK, so you are relying on ENODEV and EACCES errno's from
> DRM_XE_PERF_OP_REMOVE_CONFIG to find out (a) if OA is present and (b) if
> you need to be root (actually CAP_PERFMON or CAP_SYS_ADMIN).

yep

> 
> This logic in Xe should be close to what we have in i915? What does Mesa do
> for i915, or what doesn't work in Xe?
> 
> Here are some pointers:
> 
> * You can execute DRM_XE_DEVICE_QUERY_OA_UNITS to see if OA is present
> 
> * Add/remove OA configs and using the global OAG buffer (time based
>   sampling or DRM_XE_OA_PROPERTY_SAMPLE_OA set) are priviliged operations
>   (need root). Operations which only need OAR/OAC (OA queries, without
>   DRM_XE_OA_PROPERTY_SAMPLE_OA) can be executed by non-root.
> 
> * If "/proc/sys/dev/xe/perf_stream_paranoid" is 0, all operations can be
>   executed by non-root users. Otherwise, as I described in the previous
>   point.

It is possible that process not started by root has CAP_PERFMON:

"Unprivileged processes with enabled CAP_PERFMON capability are treated
as privileged processes with respect to perf_events performance
monitoring and observability operations,..."

And from what I understood only root can write to perf_stream_paranoid, so I don't see a point in having this file...

> 
> So basically I think you just need to check for the perf_stream_paranoid
> file above. It will tell you both (a) if OA is present (because we are
> going to merge the code which creates this file together with OA) and (b)
> if you need to be root for particular operations.
> 
> Thanks.
> --
> Ashutosh