Re: [Xen-devel] [PATCH v3] devicetree, xen: add xen, shared-memory binding

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 10/24/18 2:18 AM, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefanos@xxxxxxxxxx>
>
> Introduce a device tree binding for Xen reserved-memory regions. They
> are used to share memory across VMs from the VM config files. (See
> static_shm config option.)
>
> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> Cc: julien.grall@xxxxxxx
> ---
> Changes in v3:
> - remove fallback version
>
> Changes in v2:
> - fix Author line
> - add versioning
> - xen,id instead of id
> ---
>   .../bindings/reserved-memory/xen,shared-memory.txt   | 20 ++++++++++++++++++++
>   1 file changed, 20 insertions(+)
>   create mode 100644 
> Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt
>
> diff --git 
> a/Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt 
> b/Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt
> new file mode 100644
> index 0000000..7c81683
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt
> @@ -0,0 +1,20 @@
> +* Xen hypervisor reserved-memory binding
> +
> +Expose one or more memory regions as reserved-memory to the guest
> +virtual machine. Typically, a region is configured at VM creation time
> +to be a shared memory area across multiple virtual machines for
> +communication among them.
I may have notice some issue with this binding. Looking at the design 
document [1], the "master" domain may provide a big backing region that 
would be split and share with multiple "slave".
For the "master" domain, this binding would specify the full backing 
region. The "master" OS would not be able to know how the region would 
be used here.
For the "slave" domain, it looks like it might be possible to map the 
same region (so same ID) twice. So we would end up to create the same 
bindings twice.
Any opionion on how we should proceed with these two use cases?

> +
> +For each of these pre-shared memory regions, a range is exposed under
> +the /reserved-memory node as a child node. Each range sub-node is named
> +xen-shmem@<address> and has the following properties:
> +
> +- compatible:
> +       compatible = "xen,shared-memory-v1"
> +
> +- reg:
> +       the base guest physical address and size of the shared memory region
> +
> +- xen,id:
> +       a string that identifies the shared memory region as specified in
> +       the VM config file
Cheers,

[1] https://lists.xen.org/archives/html/xen-devel/2018-10/msg00730.html

--
Julien Grall

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