[RFC PATCH 15/15] nvme-pci: Allow mmaping the CMB in userspace

[Keith Busch]

On Fri, Nov 06, 2020 at 10:00:36AM -0700, Logan Gunthorpe wrote:
> Allow userspace to obtain CMB memory by mmaping the controller's
> char device. The mmap call allocates and returns a hunk of CMB memory,
> (the offset is ignored) so userspace does not have control over the
> address within the CMB.
> 
> A VMA allocated in this way will only be usable by drivers that set
> FOLL_PCI_P2PDMA when calling GUP. And inter-device support will be
> checked the first time the pages are mapped for DMA.
> 
> Currently this is only supported by O_DIRECT to an PCI NVMe device
> or through the NVMe passthrough IOCTL.

Rather than make this be specific to nvme, could pci p2pdma create an
mmap'able file for any resource registered with it?