linux-kernel - Re: Requirement to get BAR pci_bus

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Date:   Thu, 14 Jun 2018 20:04:53 +0000
From:   "Walker, Benjamin" <benjamin.walker@...el.com>
To:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "srinath.mannam@...adcom.com" <srinath.mannam@...adcom.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vikram.prakash@...adcom.com" <vikram.prakash@...adcom.com>,
        "okaya@...eaurora.org" <okaya@...eaurora.org>,
        "hch@....de" <hch@....de>,
        "abhishek.shah@...adcom.com" <abhishek.shah@...adcom.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-pci-owner@...r.kernel.org" <linux-pci-owner@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
Subject: Re: Requirement to get BAR pci_bus_address in user space

On Thu, 2018-06-14 at 08:50 -0600, Alex Williamson wrote:
> On Thu, 14 Jun 2018 16:18:15 +0530
> Srinath Mannam <srinath.mannam@...adcom.com> wrote:
> 
> > Hi Sinan Kaya,
> > 
> > Here are the details,
> > 
> > The issue is, For CMB cards SQs are allocated inside device BAR memory
> > which is different from normal cards.
> > In Normal cards SQ memory allocated at host side.
> > In both the cases physical address of CQ memory is programmed in NVMe
> > controller register.
> > This method works for normal cards because CQ memory is at host side.
> > But in CMB cards pci bus address equivalent to CQ memory needs to program.
> > 
> > More details are in the patch: nvme-pci: Use PCI bus address for
> > data/queues in CMB.
> > 
> > With the above patch issue is fixed in the NVMe kernel driver, But
> > similar fix is required in SPDK library also.
> > So, We need a mechanism to get pci_bus_address in user space libraries
> > to address this issue.
> 
> I don't understand the CQ vs CMB, but I think I gather that there's some
> sort of buffer that's allocated from within the devices MMIO BAR and
> some programming of the device needs to reference that buffer.
> Wouldn't you therefore use the vfio type1 IOMMU MAP_DMA ioctl to map
> the BAR into the IOVA address space and you can then use the IOVA +
> offset into the BAR for the device to reference the buffer?  It seems
> this is the same way we'd setup a peer-to-peer mapping, but we're using
> it for the device to reference itself effectively.  Thanks,

SPDK already does this today. It's capable of placing submission and completion
queues in the device's controller memory buffer (CMB) and will do all of the
right things for both uio (look up physical address) and vfio (send
IOMMU_MAP_DMA ioctl to set a iova and use that). It additionally already
supports using the CMB as a target for peer-to-peer data transfers.

This was just confirmed to work on the most recent SPDK release. If you are not
on the most recent release, I recommend trying that first. If there is still
some issue, please do take it to the SPDK mailing list.

Thanks,
Ben