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

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Date:   Tue, 18 Apr 2017 16:29:12 -0400 (EDT)
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.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
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

> On Tue, Apr 18, 2017 at 12:35 PM, Logan Gunthorpe <logang@...tatee.com>
> wrote:
> >
> >
> > On 18/04/17 01:01 PM, Jason Gunthorpe wrote:
> >> Ultimately every dma_ops will need special code to support P2P with
> >> the special hardware that ops is controlling, so it makes some sense
> >> to start by pushing the check down there in the first place. This
> >> advice is partially motivated by how dma_map_sg is just a small
> >> wrapper around the function pointer call...
> >
> > Yes, I noticed this problem too and that makes sense. It just means
> > every dma_ops will probably need to be modified to either support p2p
> > pages or fail on them. Though, the only real difficulty there is that it
> > will be a lot of work.
> 
> I don't think you need to go touch all dma_ops, I think you can just
> arrange for devices that are going to do dma to get redirected to a
> p2p aware provider of operations that overrides the system default
> dma_ops. I.e. just touch get_dma_ops().

This would not work well for everyone, for instance on GPU we usualy
have buffer object with a mix of device memory and regular system
memory but call dma sg map once for the list.

Cheers,
Jérôme