From: Jim Fehlig <email@example.com>
To: George Dunlap <firstname.lastname@example.org>, email@example.com
Cc: Ian Jackson <firstname.lastname@example.org>,
Wei Liu <email@example.com>,
Roger Pau Monne <firstname.lastname@example.org>
Subject: Re: [PATCH 7/8] docs: Document block-script protocol
Date: Wed, 16 Mar 2016 16:57:32 -0600 [thread overview]
Message-ID: <56E9E4DC.email@example.com> (raw)
On 03/16/2016 10:09 AM, George Dunlap wrote:
> Signed-off-by: George Dunlap <firstname.lastname@example.org>
> CC: Ian Jackson <email@example.com>
> CC: Wei Liu <firstname.lastname@example.org>
> CC: Roger Pau Monne <email@example.com>
> 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
> +2. When a user passes a file in the 'target' field of the disk
> +3. When a user specifies a custom script.
> +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
> +dir=$(dirname "$0")
> +. "$dir/block-common.sh"
> +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.
> + $XENBUS/mode 'r' (for readonly) or 'w' (for read-write)
> +Block scripts are responsible for making sure that if a file is
> +provided to a VM read/write, that it is not provided to any other VM.
> +FreeBSD block hotplug scripts must write
> +"$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
> +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
Xen-devel mailing list
next prev parent reply other threads:[~2016-03-16 22:57 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-16 16:09 [PATCH 0/8] tools: Allow HVM domains emulated access to disks provided by hotplug scripts George Dunlap
2016-03-16 16:09 ` [PATCH 1/8] tools/hotplug: Add a "dummy" hotplug script for testing George Dunlap
2016-03-16 16:54 ` Ian Jackson
2016-03-16 16:09 ` [PATCH 2/8] libxl: Remove redundant setting of phyical-device George Dunlap
2016-03-16 16:54 ` Ian Jackson
2016-03-16 16:09 ` [PATCH 3/8] tools/hotplug: Write physical-device-path in addition to physical-device George Dunlap
2016-03-16 16:56 ` Ian Jackson
2016-03-16 16:57 ` George Dunlap
2016-03-16 16:09 ` [PATCH 4/8] libxl: Move check for local access to a funciton George Dunlap
2016-03-16 16:58 ` Ian Jackson
2016-03-16 17:02 ` George Dunlap
2016-03-17 18:11 ` Ian Jackson
2016-03-21 15:35 ` George Dunlap
2016-03-16 16:09 ` [PATCH 5/8] libxl: Share logic for finding path between qemuu and pygrub George Dunlap
2016-03-17 18:22 ` Ian Jackson
2016-03-18 17:09 ` George Dunlap
2016-04-01 14:17 ` Ian Jackson
2016-03-16 16:09 ` [PATCH 6/8] libxl: Allow local access for block devices with hotplug scripts George Dunlap
2016-03-17 18:36 ` Ian Jackson
2016-03-18 17:17 ` George Dunlap
2016-03-16 16:09 ` [PATCH 7/8] docs: Document block-script protocol George Dunlap
2016-03-16 22:57 ` Jim Fehlig [this message]
2016-03-17 9:55 ` George Dunlap
2016-03-17 18:38 ` Ian Jackson
2016-03-16 16:09 ` [PATCH 8/8] DO NOT APPLY libxl: Change hotplug script interface to use physical-device-path George Dunlap
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