[Intel-gfx] [RFC PATCH 1/3] drm/i915/perf: allow holding preemption on filtered ctx

[Chris Wilson]

Quoting Lionel Landwerlin (2018-10-08 16:18:20)
> We would like to make use of perf in Vulkan. The Vulkan API is much
> lower level than OpenGL, with applications directly exposed to the
> concept of command buffers (pretty much equivalent to our batch
> buffers). In Vulkan, queries are always limited in scope to a command
> buffer. In OpenGL, the lack of command buffer concept meant that
> queries' duration could span multiple command buffers.
> 
> With that restriction gone in Vulkan, we would like to simplify
> measuring performance just by measuring the deltas between the counter
> snapshots written by 2 MI_RECORD_PERF_COUNT commands, rather than the
> more complex scheme we currently have in the GL driver, using 2
> MI_RECORD_PERF_COUNT commands and doing some post processing on the
> stream of OA reports, coming from the global OA buffer, to remove any
> unrelated deltas in between the 2 MI_RECORD_PERF_COUNT.
> 
> Disabling preemption only apply to a single context with which want to
> query performance counters for and is considered a privileged
> operation, by default protected by CAP_SYS_ADMIN. It is possible to
> enable it for a normal user by disabling the paranoid stream setting.

I'm am uncomfortable with disabling preemption like this. I suggest we
at least have the preemption timeout in place first.
-Chris