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

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Date:   Tue, 18 Apr 2017 15:51:27 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Tue, Apr 18, 2017 at 3:42 PM, Jason Gunthorpe
<jgunthorpe@...idianresearch.com> wrote:
> On Tue, Apr 18, 2017 at 03:28:17PM -0700, Dan Williams wrote:
>
>> Unlike the pci bus address offset case which I think is fundamental to
>> support since shipping archs do this toda
>
> But we can support this by modifying those arch's unique dma_ops
> directly.
>
> Eg as I explained, my p2p_same_segment_map_page() helper concept would
> do the offset adjustment for same-segement DMA.
>
> If PPC calls that in their IOMMU drivers then they will have proper
> support for this basic p2p, and the right framework to move on to more
> advanced cases of p2p.
>
> This really seems like much less trouble than trying to wrapper all
> the arch's dma ops, and doesn't have the wonky restrictions.

I don't think the root bus iommu drivers have any business knowing or
caring about dma happening between devices lower in the hierarchy.

>> I think it is ok to say p2p is restricted to a single sgl that gets
>> to talk to host memory or a single device.
>
> RDMA and GPU would be sad with this restriction...
>
>> That said, what's wrong with a p2p aware map_sg implementation
>> calling up to the host memory map_sg implementation on a per sgl
>> basis?
>
> Setting up the iommu is fairly expensive, so getting rid of the
> batching would kill performance..

When we're crossing device and host memory boundaries how much
batching is possible? As far as I can see you'll always be splitting
the sgl on these dma mapping boundaries.