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Re: [Xen-devel] [PATCH 8/9] drm/xen-front: Implement GEM operations

On 02/27/2018 01:47 AM, Boris Ostrovsky wrote:
On 02/23/2018 10:35 AM, Oleksandr Andrushchenko wrote:
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)

I can see how drm_info->cfg->be_alloc might be a configuration option
but xen_obj->be_alloc is set here and that's not how configuration
options typically behave.
you are right, I will put be_alloc down the code and will slightly
rework error handling for this function

+        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).

Perhaps I am missing something here but when you say "I know this can be
a problem from both security POV ..." then there is something wrong with
your solution.
well, in this scenario there are actually 2 concerns:
1. If DomU dies the pages/grants from Dom0/DomD cannot be
reclaimed back
2. Misbehaving guest may send too many requests to the
backend exhausting grant references and memory of Dom0/DomD
(this is the only concern from security POV). Please see [1]

But, we are focusing on embedded use-cases,
so those systems we use are not that "dynamic" with respect to 2).
Namely: we have fixed number of domains and their functionality
is well known, so we can do rather precise assumption on resource
usage. This is why I try to warn on such a use-case and rely on
the end user who understands the caveats

I'll probably add more precise description of this use-case
clarifying what is that security POV, so there is no confusion

Hope this explanation answers your questions

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
  * through XenStore entries.
  * For systems which do not provide IOMMU support, but having specific
  * requirements for display buffers it is possible to allocate such
  * at backend side and share those with the frontend.
  * For example, if host domain is 1:1 mapped and has DRM/GPU hardware
  * physically contiguous memory, this allows implementing zero-copying
  * use-cases.


+        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;
+    DRM_ERROR("Failed to allocate buffer with size %zu\n", size);
+    return ERR_PTR(ret);

Thank you,

[1] https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03100.html

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