linux-kernel - Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

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Date:   Thu, 1 Mar 2018 12:33:59 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Sagi Grimberg <sagi@...mberg.me>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-block@...r.kernel.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory



On 01/03/18 03:31 AM, Sagi Grimberg wrote:
>> * We also reject using devices that employ 'dma_virt_ops' which should
>>    fairly simply handle Jason's concerns that this work might break with
>>    the HFI, QIB and rxe drivers that use the virtual ops to implement
>>    their own special DMA operations.
> 
> That's good, but what would happen for these devices? simply fail the
> mapping causing the ulp to fail its rdma operation? I would think
> that we need a capability flag for devices that support it.

pci_p2pmem_find() will simply not return any devices when any client 
that uses dma_virt_ops. So in the NVMe target case it simply will not 
use P2P memory.

And just in case, pci_p2pdma_map_sg() will also return 0 if the device 
passed to it uses dma_virt_ops as well. So if someone bypasses 
pci_p2pmem_find() they will get a failure during map.

Logan