Enabling peer to peer device transactions for PCIe devices

[Dan Williams]

On Mon, Dec 5, 2016 at 9:18 AM, Jason Gunthorpe
<jgunthorpe at obsidianresearch.com> wrote:
> On Sun, Dec 04, 2016 at 07:23:00AM -0600, Stephen Bates wrote:
>> Hi All
>>
>> This has been a great thread (thanks to Alex for kicking it off) and I
>> wanted to jump in and maybe try and put some summary around the
>> discussion. I also wanted to propose we include this as a topic for LFS/MM
>> because I think we need more discussion on the best way to add this
>> functionality to the kernel.
>>
>> As far as I can tell the people looking for P2P support in the kernel fall
>> into two main camps:
>>
>> 1. Those who simply want to expose static BARs on PCIe devices that can be
>> used as the source/destination for DMAs from another PCIe device. This
>> group has no need for memory invalidation and are happy to use
>> physical/bus addresses and not virtual addresses.
>
> I didn't think there was much on this topic except for the CMB
> thing.. Even that is really a mapped kernel address..
>
>> I think something like the iopmem patches Logan and I submitted recently
>> come close to addressing use case 1. There are some issues around
>> routability but based on feedback to date that does not seem to be a
>> show-stopper for an initial inclusion.
>
> If it is kernel only with physical addresess we don't need a uAPI for
> it, so I'm not sure #1 is at all related to iopmem.
>
> Most people who want #1 probably can just mmap
> /sys/../pci/../resourceX to get a user handle to it, or pass around
> __iomem pointers in the kernel. This has been asked for before with
> RDMA.
>
> I'm still not really clear what iopmem is for, or why DAX should ever
> be involved in this..

Right, by default remap_pfn_range() does not establish DMA capable
mappings. You can think of iopmem as remap_pfn_range() converted to
use devm_memremap_pages(). Given the extra constraints of
devm_memremap_pages() it seems reasonable to have those DMA capable
mappings be optionally established via a separate driver.