[Lsf-pc] [LSF/MM/BPF proposal]: Physr discussion

[Jason Gunthorpe]

On Mon, Jan 23, 2023 at 12:50:52PM -0800, Dan Williams wrote:
> Matthew Wilcox wrote:
> > On Mon, Jan 23, 2023 at 11:36:51AM -0800, Dan Williams wrote:
> > > Jason Gunthorpe via Lsf-pc wrote:
> > > > I would like to have a session at LSF to talk about Matthew's
> > > > physr discussion starter:
> > > > 
> > > >  https://lore.kernel.org/linux-mm/YdyKWeU0HTv8m7wD@casper.infradead.org/
> > > > 
> > > > I have become interested in this with some immediacy because of
> > > > IOMMUFD and this other discussion with Christoph:
> > > > 
> > > >  https://lore.kernel.org/kvm/4-v2-472615b3877e+28f7-vfio_dma_buf_jgg@nvidia.com/
> > > 
> > > I think this is a worthwhile discussion. My main hangup with 'struct
> > > page' elimination in general is that if anything needs to be allocated
> > 
> > You're the first one to bring up struct page elimination.  Neither Jason
> > nor I have that as our motivation.
> 
> Oh, ok, then maybe I misread the concern in the vfio discussion. I
> thought the summary there is debating the ongoing requirement for
> 'struct page' for P2PDMA?

The VFIO problem is we need a unique pgmap at 4k granuals (or maybe
smaller, technically), tightly packed, because VFIO exposes PCI BAR
space that can be sized in such small amounts.

So, using struct page means some kind of adventure in the memory
hotplug code to allow tightly packed 4k pgmaps.

And that is assuming that every architecture that wants to support
VFIO supports pgmap and memory hot plug. I was just told that s390
doesn't, that is kind of important..

If there is a straightforward way to get a pgmap into VFIO then I'd do
that and give up this quest :)

I've never been looking at this from the angle of eliminating struct
page, but from the perspective of allowing the DMA API to correctly do
scatter/gather IO to non-struct page P2P memory because I *can't* get
a struct page for it. Ie make dma_map_resource() better. Make P2P
DMABUF work properly.

This has to come along with a different way to store address ranges
because the basic datum that needs to cross all the functional
boundaries we have is an address range list.

My general current sketch is we'd allocate some 'DMA P2P provider'
structure analogous to the MEMORY_DEVICE_PCI_P2PDMA pgmap and a single
provider would cover the entire MMIO aperture - eg the providing
device's MMIO BAR. This is enough information for the DMA API to do
its job.

We get this back either by searching an interval treey thing on the
physical address or by storing it directly in the address range list.

Jason