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

Dan Williams
2017-04-18T18:34:56Z

On Tue, Apr 18, 2017 at 11:00 AM, Jason Gunthorpe
<jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> On Tue, Apr 18, 2017 at 10:27:47AM -0700, Dan Williams wrote:
>> > FWIW, RDMA probably wouldn't want to use a p2mem device either, we
>> > already have APIs that map BAR memory to user space, and would like to
>> > keep using them. A 'enable P2P for bar' helper function sounds better
>> > to me.
>>
>> ...and I think it's not a helper function as much as asking the bus
>> provider "can these two device dma to each other".
>
> What I mean I could write in a RDMA driver:
>
> /* Allow the memory in BAR 1 to be the target of P2P transactions */
> pci_enable_p2p_bar(dev, 1);
>
> And not require anything else..
>
>> The "helper" is the dma api redirecting through a software-iommu
>> that handles bus address translation differently than it would
>> handle host memory dma mapping.
>
> Not sure, until we see what arches actually need to do here it is hard
> to design common helpers.
>
> Here are a few obvious things that arches will need to implement to
> support this broadly:
>
> - Virtualization might need to do a hypervisor call to get the right
> translation, or consult some hypervisor specific description table.
>
> - Anything using IOMMUs for virtualization will need to setup IOMMU
> permissions to allow the P2P flow, this might require translation to
> an address cookie.
>
> - Fail if the PCI devices are in different domains, or setup hardware to
> do completion bus/device/function translation.
>
> - All platforms can succeed if the PCI devices are under the same
> 'segment', but where segments begin is somewhat platform specific
> knowledge. (this is 'same switch' idea Logan has talked about)
>
> So, we can eventually design helpers for various common scenarios, but
> until we see what arch code actually needs to do it seems
> premature. Much of this seems to involve interaction with some kind of
> hardware, or consulation of some kind of currently platform specific
> data, so I'm not sure what a software-iommu would be doing??
>
> The main thing to agree on is that this code belongs under dma ops and
> that arches have to support struct page mapped BAR addresses in their
> dma ops inputs. Is that resonable?

I think we're saying the same thing by "software-iommu" and "custom
dma_ops", so yes.