[RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Jason Gunthorpe]

On Wed, Jan 30, 2019 at 02:22:34PM -0500, Jerome Glisse wrote:

> For GPU it would not work, GPU might want to use main memory (because
> it is running out of BAR space) it is a lot easier if the p2p_map
> callback calls the right dma map function (for page or io) rather than
> having to define some format that would pass down the information.

This is already sort of built into the sgl, you are supposed to use
is_pci_p2pdma_page() and pci_p2pdma_map_sg() and somehow it is supposed
to work out - but I think this is also fairly incomplete.

ie the current APIs seem to assume the SGL is homogeneous :(

> > Worry about optimizing away the struct page overhead later?
> 
> Struct page do not fit well for GPU as the BAR address can be reprogram
> to point to any page inside the device memory (think 256M BAR versus
> 16GB device memory).

The struct page only points to the BAR - it is not related to the
actual GPU memory in any way. The struct page is just an alternative
way to specify the physical address of the BAR page.

I think this boils down to one call to setup the entire BAR, like nvme
does, and then using the struct page in the p2p_map SGL??

Jason