Re: [PATCH 6/9] iommufd: Have pfn_reader process DMABUF iopt_pages

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Nov 20, 2025 at 08:47:20PM -0400, Jason Gunthorpe wrote:
> On Thu, Nov 20, 2025 at 08:04:37AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > Sent: Saturday, November 8, 2025 12:50 AM
> > > +
> > > +static int pfn_reader_fill_dmabuf(struct pfn_reader_dmabuf *dmabuf,
> > > +				  struct pfn_batch *batch,
> > > +				  unsigned long start_index,
> > > +				  unsigned long last_index)
> > > +{
> > > +	unsigned long start = dmabuf->start_offset + start_index * PAGE_SIZE;
> > > +
> > > +	/*
> > > +	 * This works in PAGE_SIZE indexes, if the dmabuf is sliced and
> > > +	 * starts/ends at a sub page offset then the batch to domain code will
> > > +	 * adjust it.
> > > +	 */
> > 
> > dmabuf->start_offset comes from pages->dmabuf.start, which is initialized as:
> > 
> > 	pages->dmabuf.start = start - start_byte;
> > 
> > so it's always page-aligned. Where is the sub-page offset coming from?
> 
> I need to go over this again to check it, this sub-page stuff is
> a bit convoluted. start_offset should include the sub page offset
> here..

So in the pfn reader section everything is page aligned, that means
the start location is also page aligned. The comment is just a bit
confusing, maybe this is better:

	/*
	 * start/last_index and start_offset are all PAGE_SIZE aligned, the
	 * batch is always filled using page size aligned PFNs just like the
	 * other types. If the dmabuf has been sliced with a sub page offset
	 * then the common batch to domain code will adjust it before mapping to
	 * the domain.
	 */

The adjustment is this code in batch_to_domain():

	/* The first index might be a partial page */
	if (start_index == iopt_area_index(area))
		page_offset = area->page_offset;

Thanks,
Jason