linux-kernel - Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to rdma_rw_ctx

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Date:   Mon, 8 Jan 2018 11:44:41 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 06/12] IB/core: Add optional PCI P2P flag to
 rdma_rw_ctx_[init|destroy]()



On 08/01/18 11:34 AM, Christoph Hellwig wrote:
> But P2P is _not_ a factor of the dma_ops implementation at all,
> it is something that happens behind the dma_map implementation.
> 
> Think about what the dma mapping routines do:
> 
>   (a) translate from host address to bus addresses
> 
> and
> 
>   (b) flush caches (in non-coherent architectures)
> 
> Both are obviously not needed for P2P transfers, as they never reach
> the host.

Isn't pci_p2pdma_map_sg doing (a)? It's just translating from a host 
address to a PCI bus address.

>> Very long term the IOMMUs under the ops will need to care about this,
>> so the wrapper is not an optimal place to put it - but I wouldn't
>> object if it gets it out of RDMA :)
> 
> Unless you have an IOMMU on your PCIe switch and not before/inside
> the root complex that is not correct.

Per the ACS discussion, in the future we might want to implement "ACS 
Direct Translated P2P" as Alex described. I expect it would be the IOMMU 
that needs to set that up. So, I think, we also have the dma_map 
implementations also doing something like:

(c) setup/manage any security permissions on mappings
Which P2P may at some point be concerned with.

Logan