linux-kernel - Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory

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Date:   Thu, 1 Mar 2018 23:00:51 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Logan Gunthorpe <logang@...tatee.com>
CC:     Sagi Grimberg <sagi@...mberg.me>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.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>,
        Steve Wise <swise@...ngridcomputing.com>
Subject: Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory

>> We'd prefer to have a generic way to get p2pmem instead of restricting
>> ourselves to only using CMBs. We did work in the past where the P2P memory
 >> was part of an IB adapter and not the NVMe card. So this won't work if it's
  >> an NVMe only interface.
    
 > It just seems like it it making it too complicated.

I disagree. Having a common allocator (instead of some separate allocator per driver) makes things simpler.

> Seems like a very subtle and hard to debug performance trap to leave
> for the users, and pretty much the only reason to use P2P is
> performance... So why have such a dangerous interface?

P2P is about offloading the memory and PCI subsystem of the host CPU and this is achieved no matter which p2p_dev is used.

Stephen