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

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Message-ID: <1492564984.25766.126.camel@kernel.crashing.org>
Date:   Wed, 19 Apr 2017 11:23:04 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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, 2017-04-18 at 16:24 -0600, Jason Gunthorpe wrote:
> Basically, all this list processing is a huge overhead compared to
> just putting a helper call in the existing sg iteration loop of the
> actual op.  Particularly if the actual op is a no-op like no-mmu x86
> would use.

Yes, I'm leaning toward that approach too.

The helper itself could hang off the devmap though.

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