Re: [virtio-comment] [PATCH v4 1/3] shared memory: Define shared memory regions

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 20, 2019 at 12:07:38PM +0000, Dr. David Alan Gilbert (git) 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>
> Reviewed-by: Stefan Hajnoczi <stefanha@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/conformance.tex b/conformance.tex
> index ad7e82e..f6c0d50 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -195,6 +195,7 @@ A device MUST conform to the following normative statements:
>  \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer Notification Suppression}
>  \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
>  \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Available Buffer Notification Suppression}
> +\item \ref{devicenormative:Basic Facilities of a Virtio Device / Shared Memory Regions}
>  \item \ref{devicenormative:Reserved Feature Bits}
>  \end{itemize}
>  
> diff --git a/content.tex b/content.tex
> index ede0ef6..8cd1a38 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -371,6 +371,8 @@ making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA
>  has been negotiated, these notifications would then have
>  identical \field{next_off} and \field{next_wrap} values.
>  
> +\input{shared-mem.tex}
> +
>  \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
>  
>  We start with an overview of device initialization, then expand on the
> diff --git a/shared-mem.tex b/shared-mem.tex
> new file mode 100644
> index 0000000..6dc506a
> --- /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.

Please rewrite host->device, guest->driver

> +
> +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.

I'm not sure what does above mean in context of virtio.
Is this an example?
So maybe:
	For example, with device being part of a
	hypervisor and driver being part of a VM guest
	etc etc

> +
> +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.

Let's just say in a transport-specific way.

> +
> +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 }
> +
> +References into shared memory regions are represented as offsets from
> +the beginning of the region instead of absolute memory addresses.
> +Offsets are used both for references between structures stored
> +within shared memory and for requests placed in virtqueues that
> +refer to shared memory.
> +The \field{shmid} may be explicit or may be inferred from the
> +context of the reference.
> +
> +\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.

regions must not expose regions. confused.
typo?

Also, what does stream data mean?


> +
> -- 
> 2.20.1
> 
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