[Intel-gfx] [CI 17/25] drm/i915: Assign every HW context a unique ID

[Chris Wilson]

On Thu, Apr 28, 2016 at 03:55:44PM +0100, Dave Gordon wrote:
> On 28/04/16 09:56, Chris Wilson wrote:
> >The hardware tracks contexts and expects all live contexts (those active
> >on the hardware) to have a unique identifier. This is used by the
> >hardware to assign pagefaults and the like to a particular context.
> >
> >v2: Reorder to make sure ctx->link is not left dangling if the
> >assignment of a hw_id fails (Mika).
> >
> >v3: We have 21bits of context space, not 20.
> >
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >---
> >  drivers/gpu/drm/i915/i915_debugfs.c     |  2 +-
> >  drivers/gpu/drm/i915/i915_drv.h         | 10 +++++++++
> >  drivers/gpu/drm/i915/i915_gem_context.c | 36 +++++++++++++++++++++++++++++++++
> >  3 files changed, 47 insertions(+), 1 deletion(-)
> 
> [snip]
> 
> >+static int assign_hw_id(struct drm_i915_private *dev_priv, unsigned *out)
> >+{
> >+	int ret;
> >+
> >+	ret = ida_simple_get(&dev_priv->context_hw_ida,
> >+			     0, MAX_CONTEXT_HW_ID, GFP_KERNEL);
> >+	if (ret < 0) {
> >+		/* Contexts are only released when no longer active.
> >+		 * Flush any pending retires to hopefully release some
> >+		 * stale contexts and try again.
> >+		 */
> >+		i915_gem_retire_requests(dev_priv->dev);
> >+		ret = ida_simple_get(&dev_priv->context_hw_ida,
> >+				     0, MAX_CONTEXT_HW_ID, GFP_KERNEL);
> 
> Should we once again reserve ID 0 to make it easier to identify
> things that have not yet been initialised properly?

Otoh, 0 currently is the kernel context. Since we can only get an
uninitialised value for a use-after-free, I don't see an advantage here
above and beyond using poisoning, kasan or kmemcheck.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre