Re: [Xen-devel] [PATCH v2] docs: add misc/qemu-backends.txt

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 11 Apr 2016, Juergen Gross wrote:
> Document the interface between qemu and libxl regarding backends
> supported by qemu.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>


> v2: - replace variable Xenstore path parts (<xyz>) with bash-like syntax
>       ($XYZ) as requested by Konrad Wilk
>     - add remark about de-privileged qemu as requested by Stefano
>       Stabellini
>     - add comma as suggested by Andrew Cooper
> ---
>  docs/misc/qemu-backends.txt | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>  create mode 100644 docs/misc/qemu-backends.txt
> 
> diff --git a/docs/misc/qemu-backends.txt b/docs/misc/qemu-backends.txt
> new file mode 100644
> index 0000000..b8e1799
> --- /dev/null
> +++ b/docs/misc/qemu-backends.txt
> @@ -0,0 +1,21 @@
> +In order to know whether qemu supports a specific backend type libxl
> +needs a way to obtain this information.
> +
> +As each qemu instance owns a path (named $QEMU from now on) in
> +Xenstore, the backend information is presented there. $QEMU is built
> +from the domain id where the qemu instance is running $BACKEND_DOM,
> +and the domain id of the target domain of the qemu process $DOMID:
> +
> +$QEMU = /local/domain/$BACKEND_DOM/device-model/$DOMID
> +
> +Before signalling qemu is running by writing "running" to $QEMU/state
> +qemu will create a Xenstore node for each supported backend under
> +$QEMU/backends with the backend type as name (e.g.  $QEMU/backends/qdisk
> +for the qdisk backend). In case qemu is running de-privileged (not as
> +user root) the backend nodes must be written before qemu is dropping
> +privileges.
> +
> +libxl can assume a backend of a specific type $TYPE is supported if:
> +- $QEMU/backends/$TYPE is existing in Xenstore
> +- or $QEMU/backends is not existing and $TYPE is one of:
> +  "console", "vkbd", "vfb", "qdisk", "qnic"

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