[virtio-comment] Re: [PATCH v3 1/3] shared memory: Define shared memory regions

[Cornelia Huck <cohuck@redhat.com>]

On Mon,  4 Mar 2019 13:25:29 +0000
"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:

> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Define the requirements and idea behind shared memory regions.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  conformance.tex |  1 +
>  content.tex     |  2 ++
>  shared-mem.tex  | 40 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 43 insertions(+)
>  create mode 100644 shared-mem.tex
> 
(...)
> diff --git a/shared-mem.tex b/shared-mem.tex
> new file mode 100644
> index 0000000..86b0050
> --- /dev/null
> +++ b/shared-mem.tex
> @@ -0,0 +1,40 @@
> +\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions}
> +
> +Shared memory regions are an additional facility
> +available to devices that need a region of memory that's
> +continuously shared between the host and the guest, rather
> +than passed between them in the way virtqueue elements are.
> +
> +Example uses include shared caches and version pools for versioned
> +data structures.
> +
> +The region is chosen by the host and presented to the guest, as
> +such it is useful in situations where the memory is accessed on
> +the host by other libraries that can't safely access guest RAM.
> +
> +A device may have multiple shared memory regions associated with
> +it.  Each region has a \field{shmid} to identify it, the meaning
> +of which is device-specific.
> +
> +Enumeration and location of shared memory regions is performed
> +using a transport-specific data structure and mechanism.
> +
> +Memory consistency rules vary depending on the region and the
> +device and they will be specified as required by each device.
> +
> +\subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Addressing within regions }
> +
> +Commands sent over the virtqueues may refer to data within the
> +shared memory regions, for example a command may be used by a
> +driver to cause a device to add or remove a mapping within
> +a region.  When referring to data, the addresses will normally be
> +offsets within a particular region rather than absolute host or
> +guest addresses.  The \field{shmid} may be explicit or may be
> +inferred from the command type.

It's probably reasonable to frame it like that instead of making it a
normative statement.

Maybe "the addresses are expected to be offsets"? It would feel odd if
most devices handled it like that and then an oddball device came
along...

> +
> +\devicenormative{\subsection}{Shared Memory Regions}{Basic Facilities of a Virtio
> +Device / Shared Memory Regions}
> +Shared memory regions MUST NOT expose shared memory regions which
> +are used to control the operation of the device, nor to stream
> +data.
> +

Otherwise, looks good to me.

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