Re: [virtio-dev] [PATCH v2 2/2] virtio-fs: add DAX window

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/02/19 07:33, Stefan Hajnoczi wrote:
> Describe how shared memory region ID 0 is the DAX window and how
> FUSE_SETUPMAPPING maps file ranges into the window.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  virtio-fs.tex | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/virtio-fs.tex b/virtio-fs.tex
> index ffbaa46..37ab5ea 100644
> --- a/virtio-fs.tex
> +++ b/virtio-fs.tex
> @@ -196,3 +196,28 @@ The device MUST set \field{out.unique} to 0 and set \field{out.error} to a FUSE_
>  The driver MUST verify that \field{out.unique} is 0.
>  
>  Notifications queue buffers MUST be at least 8192 bytes long.
> +
> +\subsubsection{Device Operation: DAX Window}\label{sec:Device Types / File System Device / Device Operation / Device Operation: DAX Window}
> +
> +FUSE\_READ and FUSE\_WRITE requests transfer file contents between the
> +driver-provided buffer and the device.  In cases where data transfer is
> +undesirable, the device can map file contents into the DAX window shared memory
> +region.  The driver then accesses file contents directly in device-owned memory
> +without a data transfer.
> +
> +Shared memory region ID 0 is called the DAX window.  The driver maps a file
> +range into the DAX window using the FUSE\_SETUPMAPPING request.  The mapping is
> +removed using the FUSE\_REMOVEMAPPING request.
> +
> +After FUSE\_SETUPMAPPING has completed successfully the file range is accessible
> +from the DAX window at the offset provided by the driver in the request.

FYI: this is not upstream yet, see https://lkml.org/lkml/2018/12/10/573
for a definition.

Paolo

> +\devicenormative{\paragraph}{Device Operation: DAX Window}{Device Types / File System Device / Device Operation / Device Operation: DAX Window}
> +
> +The device MUST allow mappings that completely or partially overlap existing mappings within the DAX window.
> +
> +The device MUST reject mappings that would go beyond the end of the DAX window.
> +
> +\drivernormative{\paragraph}{Device Operation: DAX Window}{Device Types / File System Device / Device Operation / Device Operation: DAX Window}
> +
> +The driver SHOULD be prepared to find shared memory region ID 0 absent and fall back to FUSE\_READ and FUSE\_WRITE requests.
>