drm/i915/userptr: add paranoid flush-on-acquire

Even though userptr objects are always coherent with the GPU, with no
way for userspace to change this with the set_caching ioctl, even on
non-LLC platforms, there is still the 'Bypass LCC' mocs setting, which
might permit reading the contents of main memory directly.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018174508.2137279-5-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_userptr.c

@@ -165,8 +165,11 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 		goto err;
 	}
 
-	sg_page_sizes = i915_sg_dma_sizes(st->sgl);
+	WARN_ON_ONCE(!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_WRITE));
+	if (i915_gem_object_can_bypass_llc(obj))
+		obj->cache_dirty = true;
 
+	sg_page_sizes = i915_sg_dma_sizes(st->sgl);
 	__i915_gem_object_set_pages(obj, st, sg_page_sizes);
 
 	return 0;