Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

From: Christoph Hellwig
Date: Fri Jun 28 2019 - 09:38:43 EST

On Thu, Jun 27, 2019 at 12:00:35PM -0600, Logan Gunthorpe wrote:
> > It is not. (c) is fundamentally very different as it is not actually
> > an operation that ever goes out to the wire at all, and which is why the
> > actual physical address on the wire does not matter at all.
> > Some interfaces like NVMe have designed it in a way that it the commands
> > used to do this internal transfer look like (b2), but that is just their
> > (IMHO very questionable) interface design choice, that produces a whole
> > chain of problems.
> >From the mapping device's driver's perspective yes, but from the
> perspective of a submitting driver they would be the same.

With your dma_addr_t scheme it won't be the same, as you'd need
a magic way to generate the internal addressing and stuff it into
the dma_addr_t. With a phys_addr_t based scheme they should basically
be all the same.

> Yes, you did suggest them. But what I'm trying to suggest is we don't
> *necessarily* need the lookup. For demonstration purposes only, a
> submitting driver could very roughly potentially do:
> struct bio_vec vec;
> dist = pci_p2pdma_dist(provider_pdev, mapping_pdev);
> if (dist < 0) {
> /* use regular memory */
> vec.bv_addr = virt_to_phys(kmalloc(...));
> vec.bv_flags = 0;
> } else if (dist & PCI_P2PDMA_THRU_HOST_BRIDGE) {
> vec.bv_addr = pci_p2pmem_alloc_phys(provider_pdev, ...);
> vec.bv_flags = BVEC_MAP_RESOURCE;
> } else {
> vec.bv_addr = pci_p2pmem_alloc_bus_addr(provider_pdev, ...);
> vec.bv_flags = BVEC_MAP_BUS_ADDR;
> }

That doesn't look too bad, except..

> -- And a mapping driver would roughly just do:
> dma_addr_t dma_addr;
> if (vec.bv_flags & BVEC_MAP_BUS_ADDR) {
> if (pci_bus_addr_in_bar(mapping_pdev, vec.bv_addr, &bar, &off)) {
> /* case (c) */
> /* program the DMA engine with bar and off */

Why bother with that here if we could also let the caller handle
that? pci_p2pdma_dist() should be able to trivially find that out
based on provider_dev == mapping_dev.

> The real difficulty here is that you'd really want all the above handled
> by a dma_map_bvec() so it can combine every vector hitting the IOMMU
> into a single continuous IOVA -- but it's hard to fit case (c) into that
> equation. So it might be that a dma_map_bvec() handles cases (a), (b1)
> and (b2) and the mapping driver has to then check each resulting DMA
> vector for pci_bus_addr_in_bar() while it is programming the DMA engine
> to deal with case (c).

I'd do it the other way around. pci_p2pdma_dist is used to find
the p2p type. The p2p type is stuff into the bio_vec, and we then:

(1) manually check for case (c) in driver for drivers that want to
treat it different from (b)
(2) we then have a dma mapping wrapper that checks the p2p type
and does the right thing for the rest.