linux-kernel - Re: [PATCH v8 13/13] nvmet: Optionally use PCI P2P memory

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Date:   Thu, 27 Sep 2018 11:12:30 -0600
From:   Keith Busch <keith.busch@...ux.intel.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     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,
        Jens Axboe <axboe@...nel.dk>,
        Christian König <christian.koenig@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Steve Wise <swise@...ngridcomputing.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 13/13] nvmet: Optionally use PCI P2P memory

On Thu, Sep 27, 2018 at 10:54:20AM -0600, Logan Gunthorpe wrote:
> We create a configfs attribute in each nvme-fabrics target port to
> enable p2p memory use. When enabled, the port will only then use the
> p2p memory if a p2p memory device can be found which is behind the
> same switch hierarchy as the RDMA port and all the block devices in
> use. If the user enabled it and no devices are found, then the system
> will silently fall back on using regular memory.
> 
> If appropriate, that port will allocate memory for the RDMA buffers
> for queues from the p2pmem device falling back to system memory should
> anything fail.
> 
> Ideally, we'd want to use an NVME CMB buffer as p2p memory. This would
> save an extra PCI transfer as the NVME card could just take the data
> out of it's own memory. However, at this time, only a limited number
> of cards with CMB buffers seem to be available.
> 
> Signed-off-by: Stephen Bates <sbates@...thlin.com>
> Signed-off-by: Steve Wise <swise@...ngridcomputing.com>
> [hch: partial rewrite of the initial code]
> Signed-off-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>

I haven't the necessary hardware to try this out, but looking forward
to it in the future. Looks good.

Reviewed-by: Keith Busch <keith.busch@...el.com>