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

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 11 Jan 2019 11:41:58 +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>
> ---
>  content.tex    |  3 +++
>  shared-mem.tex | 25 +++++++++++++++++++++++++
>  2 files changed, 28 insertions(+)
>  create mode 100644 shared-mem.tex
> 
> diff --git a/content.tex b/content.tex
> index b101d1b..321a2f4 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -331,6 +331,9 @@ Virtqueue format, or both.
>  \input{split-ring.tex}
>  
>  \input{packed-ring.tex}
> +
> +\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..6da249c
> --- /dev/null
> +++ b/shared-mem.tex
> @@ -0,0 +1,25 @@
> +\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.
> +
> +Shared memory regions MUST NOT be used to control the operation
> +of the device, nor to stream data; those should still be performed
> +using virtqueues.
> +
> +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.

"data structure and mechanism"?

> +
> +The guest physical address and the host virtual address MUST NOT
> +be used to identify structures within the memory regions; all
> +addressing MUST be relative to the start of a particular region.
> +

Is the intended implementation that the device provides a certain
memory region (in host memory) and exposes it to the driver? Are there
supposed to be any notifications of writes? Or do both simply write to
the region and get whatever updates the other side has made when they
read from the region again?

I'm a bit unsure how to implement this for the ccw transport. Maybe a
new pair of ccws to read/write shared memory regions? But we'd also
need a mechanism to discover the ids of those shared memory regions, I
think.

Halil, do you have any thoughts?