drm/i915: Fix mismatch between misplaced vma check and vma insert

When inserting a VMA, we restrict the placement to the low 4G unless the
caller opts into using the full range. This was done to allow usersapce
the opportunity to transition slowly from a 32b address space, and to
avoid breaking inherent 32b assumptions of some commands.

However, for insert we limited ourselves to 4G-4K, but on verification
we allowed the full 4G. This causes some attempts to bind a new buffer
to sporadically fail with -ENOSPC, but at other times be bound
successfully.

commit 48ea1e32 ("drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1
page") suggests that there is a genuine problem with stateless addressing
that cannot utilize the last page in 4G and so we purposefully excluded
it. This means that the quick pin pass may cause us to utilize a buggy
placement.
Reported-by: CQ Tang <cq.tang@intel.com>
Testcase: igt/gem_exec_params/larger-than-life-batch
Fixes: 48ea1e32 ("drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1 page")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: CQ Tang <cq.tang@intel.com>
Reviewed-by: CQ Tang <cq.tang@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Cc: <stable@vger.kernel.org> # v4.5+
Link: https://patchwork.freedesktop.org/patch/msgid/20201216092951.7124-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -382,7 +382,7 @@ eb_vma_misplaced(const struct drm_i915_gem_exec_object2 *entry,
 		return true;
 
 	if (!(flags & EXEC_OBJECT_SUPPORTS_48B_ADDRESS) &&
-	    (vma->node.start + vma->node.size - 1) >> 32)
+	    (vma->node.start + vma->node.size + 4095) >> 32)
 		return true;
 
 	if (flags & __EXEC_OBJECT_NEEDS_MAP &&