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

On 08/04/16 16:54, Konrad Rzeszutek Wilk wrote:
> 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?

Would you like $QEMU better? It's just a abbreviation like $HOME in

>> +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?

No, qemu is presenting the information in Xenstore.

>> +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?

What about: The Xenstore path $QEMU is built...?

>> +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?

I'm adding the information.

>> +
>> +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?

Yes. Otherwise libxl won't recognize it is running.

> 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!

This will add <domid> (or $DOMID) and <backend-dom> (or $BACKEND_DOM),
which isn't really better.


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