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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Apr 07, 2016 at 08:24:06AM +0200, Juergen Gross wrote:
> Document the interface between qemu and libxl regarding backends
> supported by qemu.


Sorry for being dense..
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  docs/misc/qemu-backends.txt | 19 +++++++++++++++++++
>  1 file changed, 19 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..f28755e
> --- /dev/null
> +++ b/docs/misc/qemu-backends.txt
> @@ -0,0 +1,19 @@
> +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

With those <> in it? Or just qemu by itself?


> +Xenstore the backend information is presented there. <qemu> is built

That looks to be missing an verb, no it has a verb, something is off with
that.

XenStore presents the backend information there?

> +from the domain id where the qemu instance is running <backend-dom>

So "<qemu> is built" .. sounds to me like you are referring to the
the variable qemu. But you are saying its value! Not the key value.

Perhaps you can sprinkle key and value in this document?

> +and the domain id of the target domain of the qemu process <domid>:
> +
> +<qemu> = /local/domain/<backend-dom>/device-model/<domid>

Ah so it is a string value which has the path to the backend.
And <qemu> is the key (why the <>?) and the value is the path?


What about persmissions? Is this suppose to seen by the frontends?
Backends? Toolstack domain?
> +
> +Before signalling qemu is running by writing "running" to <qemu>/state
> +qemu will create a Xenstore node for each supported backend under

Perhaps change this to:

It is the responsbility of QEMU to create the "qemu" key.
Also QEMU MUST write qemu/state?

Wait, is that a key under the <qemu>" path? But earlier
you mentioend that the key is a string. It can't be a path
and a key at the same time?

> +<qemu>/backends with the backend type as name (e.g.
> +<qemu>/backends/qdisk for the qdisk backend).

So <qemu> is actually a macro for the path
/local/domain<backend->dom>/device-model/<domid>!

Ah, now I get it.

I really need that coffee.

Perhaps you can replace the <qemu> by the path name? That will
make the docs longer but it will be crystal clear!

> +
> +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"
> -- 
> 2.6.6
> 
> 
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