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

On 02/28/2018 09:46 PM, Boris Ostrovsky wrote:
On 02/27/2018 01:52 AM, Oleksandr Andrushchenko wrote:
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:
+        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

How will dom0/backend know whether or not to trust the front end (and
thus whether or not to provide provide pages to it)? Will there be
something in xenstore, for example, to indicate such trusted frontends?
Exactly, there is a dedicated xl configuration option available [1] for vdispl:

Indicates if backend can be a buffer provider/allocator for this domain. See display protocol for details."

Thus, one can configure this per domain for trusted ones in corresponding
xl configuration files

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://xenbits.xen.org/docs/4.10-testing/man/xl.cfg.5.html

   Indicates if backend can be a buffer provider/allocator for this
   domain. See display protocol for details.

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