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    SubjectRe: [PATCH 1/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver
    On Fri, Apr 06, 2018 at 02:25:08PM +0300, Oleksandr Andrushchenko wrote:
    > On 04/03/2018 12:47 PM, Daniel Vetter wrote:
    > > On Thu, Mar 29, 2018 at 04:19:31PM +0300, Oleksandr Andrushchenko wrote:
    > > > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
    > > > +static int to_refs_grant_foreign_access(struct xen_gem_object *xen_obj)
    > > > +{
    > > > + grant_ref_t priv_gref_head;
    > > > + int ret, j, cur_ref, num_pages;
    > > > + struct sg_page_iter sg_iter;
    > > > +
    > > > + ret = gnttab_alloc_grant_references(xen_obj->num_pages,
    > > > + &priv_gref_head);
    > > > + if (ret < 0) {
    > > > + DRM_ERROR("Cannot allocate grant references\n");
    > > > + return ret;
    > > > + }
    > > > +
    > > > + j = 0;
    > > > + num_pages = xen_obj->num_pages;
    > > > + for_each_sg_page(xen_obj->sgt->sgl, &sg_iter, xen_obj->sgt->nents, 0) {
    > > > + struct page *page;
    > > > +
    > > > + page = sg_page_iter_page(&sg_iter);
    > > Quick drive-by: You can't assume that an sgt is struct page backed.
    > Do you mean that someone could give me sgt which never
    > seen sg_assign_page for its entries?

    Yes.

    > What are the other use-cases for that to happen?

    Sharing vram or other resources which are not backed by a struct page. See
    Christian König's recent work to accomplish just that for admgpu.

    > > And you probably want to check this at import/attach time.
    > The check you mean is to make sure that when I call
    > page = sg_page_iter_page(&sg_iter);
    > I have to make sure that I get a valid page?

    Yup.

    > > -Daniel
    > Thank you,
    > Oleksandr

    Cheers, Daniel
    --
    Daniel Vetter
    Software Engineer, Intel Corporation
    http://blog.ffwll.ch

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