Re: [PATCH v2 8/9] docs: Document block-script protocol

[George Dunlap <dunlapg@umich.edu>]

On Wed, Mar 23, 2016 at 12:14 PM, Roger Pau Monné <roger.pau@citrix.com> wrote:
> On Wed, 23 Mar 2016, George Dunlap wrote:
>> On 22/03/16 12:52, Roger Pau Monné wrote:
>> > On Mon, 21 Mar 2016, George Dunlap wrote:
>> >> Signed-off-by: George Dunlap <george.dunlap@citrix.com>
>> >> ---
>> >> Changes since v1:
>> >> - Attempt to make a clear distinction between custom hotplug scripts
>> >> and the script called for raw physical devices and files
>> >>
>> >> CC: Ian Jackson <ian.jackson@citrix.com>
>> >> CC: Wei Liu <wei.liu2@citrix.com>
>> >> CC: Roger Pau Monne <roger.pau@citrix.com>
>> >> ---
>> >>  docs/misc/block-scripts.txt | 101 ++++++++++++++++++++++++++++++++++++++++++++
>> >>  1 file changed, 101 insertions(+)
>> >>
>> >> diff --git a/docs/misc/block-scripts.txt b/docs/misc/block-scripts.txt
>> >> new file mode 100644
>> >> index 0000000..6dd5d48
>> >> --- /dev/null
>> >> +++ b/docs/misc/block-scripts.txt
>> >> @@ -0,0 +1,101 @@
>> > [...]
>> >> +Inputs
>> >> +------
>> >> +
>> >> +In all cases, the scripts are called with either "add" or "remove" as
>> >> +the command.  For custom scripts, the command will be the first
>> >> +argument of the script (i.e. $1).
>> >> +
>> >> +The environment variable XENBUS_PATH will be set to the
>> >> +path for the block device to be created.
>> >
>> > This is true for Linux, but not for NetBSD. On NetBSD no env variables are
>> > needed, and everything is passed as arguments using the following format:
>> >
>> > ./<script> <backend_path> <xenbus state>
>> >
>> > Where xenbus state is either 2 or 6.
>> >
>> > On FreeBSD I'm aiming of using the same input interface for both block and
>> > network scripts, and it is the following:
>> >
>> > ./<script> {add/remove} <backend path>
>> >
>> > With no env variables provided at all. So either this section is expanded,
>> > or it is labelled as "Linux Inputs".
>>
>> Nothing like consistency across implementations. :-)
>>
>> So in the case of NetBSD, "2" means 'add' and "6" means 'remove'?  Or
>> how does that work?
>
> Yes, 2 means add and 6 remove.
>
>> Presumably there's not much we can do about NetBSD at this point, if
>> there are (or may be) out-of-tree scripts that expect the new format.
>> But unless there's a good reason, it seems like we should try to
>> converge the hotplug script protocol.
>>
>> Was there a particular reason you wanted to use an argument instead of
>> an environment variable?  If not, it's probably better to just follow
>> suit with the Linux protocol.
>
> Don't get me wrong, but the Linux protocol is all but consistent :). I'm
> not sure if those other env variables are used by the block hotplug
> scripts, but we also set:
>
> script=<script_name>
> XENBUS_TYPE=<vbd/vif>
> XENBUS_PATH=<be_path>
> XENBUS_BASE_PATH="backend"
>
> And it's even worse for vifs, where the action parameter that we pass to
> the hotplug script is different depending on whether we are dealing with
> a PV or an emulated interface (PV uses "online/offline", while emulated
> use "add/remove").
>
> I would like to share the same interface, but I think the Linux one is
> simply too broken, and I don't want to put this anywhere close to FreeBSD.
> There at least I have a chance of having something that's simple and
> rational.

OK, so how about this.  I'll post the document with the input sections
like this:
[snip]

Inputs
------

Unfortunately the inputs to the block scripts look completely
different for each operating system.

Inputs (Linux)
--------------

In all cases, the scripts are called with either "add" or "remove" as
the command.  For custom scripts, the command will be the first
argument of the script (i.e. $1).

The environment variable XENBUS_PATH will be set to the
path for the block device to be created.

When the script is run, the following nodes shall already have been
written into xenstore:

 $XENBUS/params    The contents of the 'target' section of the disk
specification verbatim.
 $XENBUS/mode      'r' (for readonly) or 'w' (for read-write)

Inputs (NetBSD)
---------------

TODO

[snip]

And you add a FreeBSD section in your series.  If that sounds good
I'll make the above modification and send v3.

 -George

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