[Intel-gfx] [PATCH 17/37] drm/i915: Track vma activity per fence.context, not per engine

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-06-29 16:08:40)
> 
> On 29/06/2018 15:54, Tvrtko Ursulin wrote:
> 
> [snip]
> 
> >> +int i915_vma_move_to_active(struct i915_vma *vma,
> >> +                struct i915_request *rq,
> >> +                unsigned int flags)
> >> +{
> >> +    struct drm_i915_gem_object *obj = vma->obj;
> >> +    struct i915_gem_active *active;
> >> +
> >> +    lockdep_assert_held(&rq->i915->drm.struct_mutex);
> >> +    GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
> >> +
> >> +    active = lookup_active(vma, rq->fence.context);
> > 
> > Never mind the radix tree, but fence.context is u64 as well. And 
> > assigned values are continuously incrementing so once >4G of contexts 
> > are created and destroyed aliasing is guaranteed with the kernel 
> > context, or any old one.
> > 
> > It is probably IGT abuse territory, but a) can we be sure it will not 
> > open up some exploit, and b) can we swallow this problem ourselves?
> 
> Hm.. key the radix tree with the timeline pointer instead? 1:1 to 
> fence.context, natural long, and automatic lifetime management.

Lets see.
-Chris