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

[Logan Gunthorpe]

On 05/01/18 11:11 AM, Keith Busch wrote:
> 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.

Although it is not explicitly stated anywhere, pci_alloc_p2pmem() should 
always be at least 4k aligned. This is because the gen_pool that 
implements it is created with PAGE_SHIFT for its min_alloc_order.

Logan