Re: [PATCH 1/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

[Daniel Vetter <daniel@ffwll.ch>]

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