On 02/23/2018 05:26 PM, Boris Ostrovsky wrote:
On 02/21/2018 03:03 AM, Oleksandr Andrushchenko wrote:
+static struct xen_gem_object *gem_create(struct drm_device *dev, size_t size)
+{
+       struct xen_drm_front_drm_info *drm_info = dev->dev_private;
+       struct xen_gem_object *xen_obj;
+       int ret;
+
+       size = round_up(size, PAGE_SIZE);
+       xen_obj = gem_create_obj(dev, size);
+       if (IS_ERR_OR_NULL(xen_obj))
+               return xen_obj;
+
+       if (drm_info->cfg->be_alloc) {
+               /*
+                * backend will allocate space for this buffer, so
+                * only allocate array of pointers to pages
+                */
+               xen_obj->be_alloc = true;
If be_alloc is a flag (which I am not sure about) --- should it be set
to true *after* you've successfully allocated your things?
this is a configuration option telling about the way
the buffer gets allocated: either by the frontend or
backend (be_alloc -> buffer allocated by the backend)


+               ret = gem_alloc_pages_array(xen_obj, size);
+               if (ret < 0) {
+                       gem_free_pages_array(xen_obj);
+                       goto fail;
+               }
+
+               ret = alloc_xenballooned_pages(xen_obj->num_pages,
+                               xen_obj->pages);
Why are you allocating balloon pages?
in this use-case we map pages provided by the backend
(yes, I know this can be a problem from both security
POV and that DomU can die holding pages of Dom0 forever:
but still it is a configuration option, so user decides
if her use-case needs this and takes responsibility for
such a decision).

Please see description of the buffering modes in xen_drm_front.h
specifically for backend allocated buffers:
 *******************************************************************************
 * 2. Buffers allocated by the backend
 *******************************************************************************
 *
 * This mode of operation is run-time configured via guest domain configuration
 * through XenStore entries.
 *
 * For systems which do not provide IOMMU support, but having specific
 * requirements for display buffers it is possible to allocate such buffers
 * at backend side and share those with the frontend.
 * For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting
 * physically contiguous memory, this allows implementing zero-copying
 * use-cases.


-boris

+               if (ret < 0) {
+                       DRM_ERROR("Cannot allocate %zu ballooned pages: %d\n",
+                                       xen_obj->num_pages, ret);
+                       goto fail;
+               }
+
+               return xen_obj;
+       }
+       /*
+        * need to allocate backing pages now, so we can share those
+        * with the backend
+        */
+       xen_obj->num_pages = DIV_ROUND_UP(size, PAGE_SIZE);
+       xen_obj->pages = drm_gem_get_pages(&xen_obj->base);
+       if (IS_ERR_OR_NULL(xen_obj->pages)) {
+               ret = PTR_ERR(xen_obj->pages);
+               xen_obj->pages = NULL;
+               goto fail;
+       }
+
+       return xen_obj;
+
+fail:
+       DRM_ERROR("Failed to allocate buffer with size %zu\n", size);
+       return ERR_PTR(ret);
+}
+


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