linux-kernel - Re: [RFC 6/8] nvmet: Be careful about using iomem accesses when dealing with p2pmem

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Date:   Tue, 4 Apr 2017 11:21:58 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Sagi Grimberg <sagi@...mberg.me>
Cc:     Christoph Hellwig <hch@....de>,
        "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>,
        Dan Williams <dan.j.williams@...el.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org, linux-nvdimm@...1.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 6/8] nvmet: Be careful about using iomem accesses when
 dealing with p2pmem



On 04/04/17 04:59 AM, Sagi Grimberg wrote:
> We can never ever get here from an IO command, and that is a good thing
> because it would have been broken if we did, regardless of what copy
> method we use...

Yes, I changed this mostly for admin commands. I did notice connect
commands do end up reading from the p2mem and this patchset correctly
switches it to iomemcpy. However, based on Cristoph's comment, I hope to
make it more general such that iomem is hidden within sgls and any
access will either be correct or create a warning.


On 04/04/17 09:46 AM, Jason Gunthorpe wrote:
> Transactions might not complete at the NVMe device before the CPU
> processes the RDMA completion, however due to the PCI-E ordering rules
> new TLPs directed to the NVMe will complete after the RMDA TLPs and
> thus observe the new data. (eg order preserving)
> 
> It would be very hard to use P2P if fabric ordering is not preserved..

Yes, my understanding is the same, the PCI-E ordering rules save us here.

Thanks,

Logan