+/**
+ * DOC: Virtual Engine uAPI
+ *
+ * Virtual engine is a concept where userspace is able to configure a set of
+ * physical engines, submit a batch buffer, and let the driver execute it on any
+ * engine from the set as it sees fit.
+ *
+ * This is primarily useful on parts which have multiple instances of a same
+ * class engine, like for example GT3+ Skylake parts with their two VCS engines.
+ *
+ * For instance userspace can enumerate all engines of a certain class using the
+ * previously described `Engine Discovery uAPI`_. After that userspace can
+ * create a GEM context with a placeholder slot for the virtual engine (using
+ * `I915_ENGINE_CLASS_INVALID` and `I915_ENGINE_CLASS_INVALID_NONE` for class
+ * and instance respectively) and finally using the
+ * `I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE` extension place a virtual engine in
+ * the same reserved slot.
+ *
+ * Example of creating a virtual engine and submitting a batch buffer to it:
+ *
+ * .. code-block:: C
+ *
+ * I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(virtual, 2) = {
+ * .base.name = I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE,
+ * .engine_index = 0, // Place this virtual engine into engine map slot 0
+ * .num_siblings = 2,
+ * .engines = { { I915_ENGINE_CLASS_VIDEO, 0 },
+ * { I915_ENGINE_CLASS_VIDEO, 1 }, },
+ * };
+ * I915_DEFINE_CONTEXT_PARAM_ENGINES(engines, 1) = {
+ * .engines = { { I915_ENGINE_CLASS_INVALID,
+ * I915_ENGINE_CLASS_INVALID_NONE } },
+ * .extensions = to_user_pointer(&virtual), // Chains after load_balance extension
+ * };
+ * struct drm_i915_gem_context_create_ext_setparam p_engines = {
+ * .base = {
+ * .name = I915_CONTEXT_CREATE_EXT_SETPARAM,
+ * },
+ * .param = {
+ * .param = I915_CONTEXT_PARAM_ENGINES,
+ * .value = to_user_pointer(&engines),
+ * .size = sizeof(engines),
+ * },
+ * };
+ * struct drm_i915_gem_context_create_ext create = {
+ * .flags = I915_CONTEXT_CREATE_FLAGS_USE_EXTENSIONS,
+ * .extensions = to_user_pointer(&p_engines);
+ * };
+ *
+ * ctx_id = gem_context_create_ext(drm_fd, &create);
+ *
+ * // Now we have created a GEM context with its engine map containing a
+ * // single virtual engine. Submissions to this slot can go either to
+ * // vcs0 or vcs1, depending on the load balancing algorithm used inside
+ * // the driver. The load balancing is dynamic from one batch buffer to
+ * // another and transparent to userspace.
+ *
+ * ...
+ * execbuf.rsvd1 = ctx_id;
+ * execbuf.flags = 0; // Submits to index 0 which is the virtual engine
+ * gem_execbuf(drm_fd, &execbuf);
+ */
+