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To:     Sagi Grimberg <sagi@grimberg.me>, linux-kernel@vger.kernel.org,
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 <90b658d9-da8a-af36-ad8f-a72ef0c6e3c1@grimberg.me>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <112b6d99-8aac-3d45-486e-ebea62fb8670@deltatee.com>
Date:   Thu, 1 Mar 2018 12:33:59 -0700
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Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory
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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