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

[Keith Busch <keith.busch-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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.