Re: [PATCH v6 4/5] fs, xfs: introduce MAP_DIRECT for creating block-map-atomic file ranges

[Christoph Hellwig]

On Thu, Aug 24, 2017 at 09:31:17AM -0700, Dan Williams wrote:
> External agent is a DMA device, or a hypervisor like Xen. In the DMA
> case perhaps we can use the fcntl lease mechanism, I'll investigate.
> In the Xen case it actually would need to use fiemap() to discover the
> physical addresses that back the file to setup their M2P tables.
> Here's the discussion where we discovered that physical address
> dependency:
> 
>     https://lists.xen.org/archives/html/xen-devel/2017-04/msg00419.html

fiemap does not work to discover physical addresses.  If they want
to do anything involving physical address they will need a kernel
driver.