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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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
> -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