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

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Date:   Wed, 19 Apr 2017 13:31:54 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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 Wed, Apr 19, 2017 at 01:02:49PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 19/04/17 12:32 PM, Jason Gunthorpe wrote:
> > On Wed, Apr 19, 2017 at 12:01:39PM -0600, Logan Gunthorpe wrote:
> > Not entirely, it would have to call through the whole process
> > including the arch_p2p_cross_segment()..
> 
> Hmm, yes. Though it's still not clear what, if anything,
> arch_p2p_cross_segment would be doing.

Sets up the iommu for arches that place a iommu between the pci root
port and other pci root ports.

> In my experience, if you are going between host bridges, the CPU
> address (or PCI address -- I'm not sure which seeing they are the
> same on my system) would still work fine

Try it with VT-D turned on. It shouldn't work or there is a notable
security hole in your platform..

> > 	 const struct dma_map_ops *comp_ops = get_dma_ops(completer);
> > 	 const struct dma_map_ops *init_ops = get_dma_ops(initiator);
>
> So, in this case, what device does the completer point to? The PCI
> device or a more specific GPU device? If it's the former, who's
> responsible for setting the new dma_ops? Typically the dma_ops are arch
> specific but now you'd be adding ones that are tied to hmm or the gpu.

Donno, that is for GPU folks to figure out :)

But.. it could point to a GPU and the GPU struct device could have a
proxy dma_ops like Dan pointed out.

> >> I'm not sure I like the name pci_p2p_same_segment. It reads as though
> >> it's only checking if the devices are not the same segment.
> > 
> > Well, that is exactly what it is doing. If it succeeds then the caller
> > knows the DMA will not flow outside the segment and no iommu setup/etc
> > is required.
> 
> It appears to me like it's calculating the DMA address, and the check is
> just a side requirement. It reads as though it's only doing the check.

pci_p2p_same_segment_get_pa() then?

Jason