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

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Message-ID: <6c89e4b4-1854-c251-a5ec-7e54bc8085fc@deltatee.com>
Date:   Fri, 5 Jan 2018 11:19:28 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Keith Busch <keith.busch@...el.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 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