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Subject: Re: [PATCH v5 2/2] virtio-fs: add DAX window

On Fri, Jul 26, 2019 at 10:53:38AM +0100, 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>
> ---

Are there any security considerations with this memory sharing?
If yes it might be a good idea to add a chapter about that,
and functionality/performance/security tradeoffs involved.

> Note that this depends on the shared memory resource specification
> extension that David Gilbert is working on.
> https://lists.oasis-open.org/archives/virtio-comment/201901/msg00000.html
> The FUSE_SETUPMAPPING message is part of the virtio-fs Linux patches:
> https://gitlab.com/virtio-fs/linux/blob/virtio-fs/include/uapi/linux/fuse.h
> ---
>  virtio-fs.tex | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> diff --git a/virtio-fs.tex b/virtio-fs.tex
> index a9f47ce..2d01a51 100644
> --- a/virtio-fs.tex
> +++ b/virtio-fs.tex
> @@ -170,6 +170,51 @@ \subsubsection{Device Operation: High Priority Queue}\label{sec:Device Types / F
>  The driver MUST anticipate that request queues are processed concurrently with the hiprio queue.
> +\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.  Drivers map this shared
> +memory region with writeback caching as if it were regular RAM.  The contents
> +of the DAX window are undefined unless a mapping exists for that range.
> +
> +The driver maps a file range into the DAX window using the FUSE\_SETUPMAPPING
> +request.  Alignment constraints for FUSE\_SETUPMAPPING and FUSE\_REMOVEMAPPING
> +requests are communicated during FUSE\_INIT negotiation.
> +
> +When a FUSE\_SETUPMAPPING request perfectly overlaps a previous mapping, the
> +previous mapping is replaced.  When a mapping partially overlaps a previous
> +mapping, the previous mapping is split into one or two smaller mappings.  When
> +a mapping is partially unmapped it is also split into one or two smaller
> +mappings.
> +
> +Establishing new mappings or splitting existing mappings consumes resources.
> +If the device runs out of resources the FUSE\_SETUPMAPPING request fails until
> +resources are available again following FUSE\_REMOVEMAPPING.
> +
> +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.  A mapping is removed using the FUSE\_REMOVEMAPPING request.
> +
> +Data is only guaranteed to be persistent when a FUSE\_FSYNC request is used by
> +the device after having been made available by the driver following the write.
> +
> +\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.
> +
> +The driver MUST NOT access DAX window areas that have not been mapped.
> +
>  \subsubsection{Security Considerations}\label{sec:Device Types / File System Device / Security Considerations}
>  The device provides access to a file system containing files owned by one or
> -- 
> 2.21.0

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