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Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>,
        Logan Gunthorpe
	 <logang@deltatee.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
        Bjorn Helgaas
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        Sagi Grimberg
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        Jens Axboe
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        linux-scsi
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jerome
 Glisse <jglisse@redhat.com>
Date: Wed, 19 Apr 2017 11:23:04 +1000
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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