linux-kernel - Re: [PATCH 09/12] nvme-pci: Use PCI p2pmem subsystem to manage the CMB

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Date:   Fri, 5 Jan 2018 11:11:53 -0700
From:   Keith Busch <keith.busch@...el.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,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Sagi Grimberg <sagi@...mberg.me>,
        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>
Subject: Re: [PATCH 09/12] nvme-pci: Use PCI p2pmem subsystem to manage the
 CMB

On Thu, Jan 04, 2018 at 12:01:34PM -0700, Logan Gunthorpe wrote:
> Register the CMB buffer as p2pmem and use the appropriate allocation
> functions to create and destroy the IO SQ.
> 
> If the CMB supports WDS and RDS, publish it for use as p2p memory
> by other devices.

<>

> +	if (qid && dev->cmb_use_sqes) {
> +		nvmeq->sq_cmds = pci_alloc_p2pmem(pdev, SQ_SIZE(depth));

> +		nvmeq->sq_dma_addr = pci_p2pmem_virt_to_bus(pdev,
> +							    nvmeq->sq_cmds);
> +		nvmeq->sq_cmds_is_io = true;
> +	}

This gets into some spec type trouble for creating an IO submission
queue. We use the sq_dma_addr as the PRP entry to the Create I/O
SQ command, which has some requirements:

  "the PRP Entry shall have an offset of 0h."

So this has to be 4k aligned, but pci_alloc_p2pmem doesn't guarantee
that. I doubt the spec's intention was to require such alignment for
CMB SQs, but there may be controllers enforcing the rule as written.