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

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Date:   Tue, 18 Apr 2017 17:03:59 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Dan Williams <dan.j.williams@...el.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 18/04/17 04:24 PM, Jason Gunthorpe wrote:
> Try and write a stacked map_sg function like you describe and you will
> see how horrible it quickly becomes.

Yes, unfortunately, I have to agree with this statement completely.

> Since dma mapping is a performance path we must be careful not to
> create intrinsic inefficiencies with otherwise nice layering :)

Yeah, I'm also personally thinking your proposal is the way to go as
well. Dan's injected ops suggestion is interesting but I can't see how
it solves the issue completely. Your proposal is the only one that seems
to be complete to me. It just has a few minor pain points which I've
already described but are likely manageable and less than the pain
stacked dma_ops creates.

Logan