Re: [Xen-devel] [PATCH 7/8] docs: Document block-script protocol

[Jim Fehlig <jfehlig@xxxxxxxx>]

On 03/16/2016 10:09 AM, George Dunlap wrote:
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> ---
>  docs/misc/block-scripts.txt | 100 
> ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 100 insertions(+)
>
> diff --git a/docs/misc/block-scripts.txt b/docs/misc/block-scripts.txt
> new file mode 100644
> index 0000000..ef19207
> --- /dev/null
> +++ b/docs/misc/block-scripts.txt
> @@ -0,0 +1,100 @@
> +Block scripts
> +=============
> +
> +Block scripts are called at the moment anytime blkback is directly
> +involved in providing access to a backend.  There are three general
> +cases this happens:
> +
> +1. When a user passes a block device in the 'target' field of the disk
> +specification
> +
> +2. When a user passes a file in the 'target' field of the disk
> +specification
> +
> +3. When a user specifies a custom script.
> +
> +Setup
> +-----
> +
> +It is highly recommended that custom scripts as much as possible
> +include and use the common Xen functionality.  If the script is run
> +from the normal block script location (/etc/xen/scripts by default),
> +then this can be done by adding the following to the top of the
> +script:
> +
> +dir=$(dirname "$0")
> +. "$dir/block-common.sh"
> +
> +
> +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.
> +
> +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.
> + $XENBUS/mode      'r' (for readonly) or 'w' (for read-write)
> +
> +Output
> +-------
> +
> +Block scripts are responsible for making sure that if a file is
> +provided to a VM read/write, that it is not provided to any other VM.
> +
> +FreeBSD block hotplug scripts must write
> +"$XENBUS_PATH/physical-device-path" with the path to the physical
> +device or file.  Linux and NetBSD block hotplug scripts *should* also
> +write this node.
> +
> +For the time being, Linux and NetBSD block hotplug scripts must write
> +"$XENBUS_PATH/physical-device" with the device's major and minor
> +numbers, written in hex, and separated by a colon.
> +
> +Scripts which include block-common.sh can simply call write_dev "$dev"
> +with a path to the device, and write_dev will do the right thing, now
> +and going forward.  (See the discussion below.)
> +
> +Rationale and future work
> +-------------------------
> +
> +Historically, the block scripts wrote a node called "physical-device",
> +which contains the major and minor numbers, written in hex, and
> +separated by a colon (e.g., "1a:2").  This is required by the Linux
> +blkback driver.
> +
> +FreeBSD blkback, on the other hand, does not have the concept of
> +major/minor numbers, and can give direct access to a file without
> +going through loopback; so its driver will consume
> +physical-device-path.
> +
> +On Linux, the device model (qemu) needs access to a file it can
> +interpret to provide emulated disks before paravirtualized drivers are
> +marked as up.  The easiest way to accomplish this is to allow qemu to
> +consume physical-device-path (rather than, say, having dom0 act as
> +both a frontend and a backend).
> +
> +Going forward, the plan is at some point to have all block scripts
> +simply write "physical-device-path", and then have libxl write the
> +other nodes.  The reason we haven't done this yet is that the main
> +block script wants to check to make sure the *major/minor* number
> +hasn't been re-used, rather than just checking that the *specific
> +device node* isn't re-used.  To do this it currently uses
> +physical-device; and to do this *safely* it needs physical-device to
> +be written with the lock held.
> +
> +The simplest solution for sorting this out would be to have the block
> +script use physical-device if it's present, but if not, to directly
> +stat physical-device-path.  But there's not time before the 4.7
> +release to make sure all that works.
> +
> +Another possibility would be to do away with the block scripts
> +altogether when not actually running any scripts,

Just to clarify, do you mean drop support for all block scripts?

>  and do the duplicate
> +checking inside of libxl.  The rationale for doing this in block
> +scripts rather than in libxl isn't clear at thes point.

Block scripts like block-iscsi and block-drbd also "cook" $XENBUS/params into
$XENBUS_PATH/physical-device{,-path} right?

Regards,
Jim

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