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

[Boris Ostrovsky]

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.


>>
>>> +        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.

-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);
>>> +}
>>> +
>>>
>