Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

[Benjamin Herrenschmidt]

On Thu, 2017-04-13 at 15:22 -0600, Logan Gunthorpe wrote:
> 
> On 12/04/17 03:55 PM, Benjamin Herrenschmidt wrote:
> > Look at pcibios_resource_to_bus() and pcibios_bus_to_resource(). They
> > will perform the conversion between the struct resource content (CPU
> > physical address) and the actual PCI bus side address.
> 
> Ah, thanks for the tip! On my system, this translation returns the same
> address so it was not necessary. And, yes, that means this would have to
> find its way into the dma mapping routine somehow. This means we'll
> eventually need a way to look-up the p2pmem device from the struct page.
> Which means we will likely need a new flag bit in the struct page or
> something. The big difficulty I see is testing. Do you know what
> architectures or in what circumstances are these translations used?

I think a bunch of non-x86 architectures but I don't know which ones
outside of powerpc.

> > When behind the same switch you need to use PCI addresses. If one tries
> > later to do P2P between host bridges (via the CPU fabric) things get
> > more complex and one will have to use either CPU addresses or something
> > else alltogether (probably would have to teach the arch DMA mapping
> > routines to work with those struct pages you create and return the
> > right thing).
> 
> Probably for starters we'd want to explicitly deny cases between host
> bridges and add that later if someone wants to do the testing.

Cheers,
Ben.

> Thanks,
> 
> Logan