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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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.