Messages in this thread | | | Subject | Re: [PATCH 8/9] drm/xen-front: Implement GEM operations | From | Oleksandr Andrushchenko <> | Date | Wed, 28 Feb 2018 21:52:56 +0200 |
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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:
"be-alloc=BOOLEAN 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 > -boris > > >> 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 >>> -boris >>> >>>> 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); >>>>>> +} >>>>>> + >>>>>> >> Thank you, >> Oleksandr >> >> [1] >> https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03100.html >> [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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