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George Dunlap
1129a99d2f
Signed-off-by: George Dunlap <george.dunlap@citrix.com> Acked-by: Ian Jackson <ian.jackson@eu.citrix.com> Tested-by: George Dunlap <george.dunlap@citrix.com>
113 lines
3.9 KiB
Plaintext
113 lines
3.9 KiB
Plaintext
Block scripts
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=============
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Block scripts are called at the moment anytime blkback is directly
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involved in providing access to a backend. There are three general
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cases this happens:
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1. When a user passes a block device in the 'target' field of the disk
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specification
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2. When a user passes a file in the 'target' field of the disk
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specification
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3. When a user specifies a custom hotplug script.
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Setup
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-----
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It is highly recommended that custom hotplug scripts as much as
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possible include and use the common Xen functionality. If the script
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is run from the normal block script location (/etc/xen/scripts by
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default), then this can be done by adding the following to the top of
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the script:
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dir=$(dirname "$0")
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. "$dir/block-common.sh"
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Inputs
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------
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Unfortunately the inputs to the block scripts look completely
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different for each operating system.
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Inputs (Linux)
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--------------
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In all cases, the scripts are called with either "add" or "remove" as
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the command. For custom scripts, the command will be the first
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argument of the script (i.e. $1).
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The environment variable XENBUS_PATH will be set to the
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path for the block device to be created.
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When the script is run, the following nodes shall already have been
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written into xenstore:
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$XENBUS/params The contents of the 'target' section of the disk specification verbatim.
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$XENBUS/mode 'r' (for readonly) or 'w' (for read-write)
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Inputs (NetBSD)
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---------------
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TODO
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Output
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-------
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Block scripts are responsible for making sure that if a file is
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provided to a VM read/write, that it is not provided to any other VM.
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FreeBSD block hotplug scripts must write
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"$XENBUS_PATH/physical-device-path" with the path to the physical
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device or file. Linux and NetBSD block hotplug scripts *should* also
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write this node.
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For the time being, Linux and NetBSD block hotplug scripts must write
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"$XENBUS_PATH/physical-device" with the device's major and minor
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numbers, written in hex, and separated by a colon.
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Scripts which include block-common.sh can simply call write_dev "$dev"
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with a path to the device, and write_dev will do the right thing, now
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and going forward. (See the discussion below.)
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Rationale and future work
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-------------------------
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Historically, the block scripts wrote a node called "physical-device",
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which contains the major and minor numbers, written in hex, and
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separated by a colon (e.g., "1a:2"). This is required by the Linux
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blkback driver.
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FreeBSD blkback, on the other hand, does not have the concept of
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major/minor numbers, and can give direct access to a file without
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going through loopback; so its driver will consume
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physical-device-path.
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On Linux, the device model (qemu) needs access to a file it can
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interpret to provide emulated disks before paravirtualized drivers are
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marked as up. The easiest way to accomplish this is to allow qemu to
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consume physical-device-path (rather than, say, having dom0 act as
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both a frontend and a backend).
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Going forward, the plan is at some point to have all block scripts
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simply write "physical-device-path", and then have libxl write the
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other nodes. The reason we haven't done this yet is that the main
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block script wants to check to make sure the *major/minor* number
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hasn't been re-used, rather than just checking that the *specific
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device node* isn't re-used. To do this it currently uses
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physical-device; and to do this *safely* it needs physical-device to
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be written with the lock held.
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The simplest solution for sorting this out would be to have the block
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script use physical-device if it's present, but if not, to directly
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stat physical-device-path. But there's not time before the 4.7
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release to make sure all that works.
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Another possibility would be to only call out to scripts when using
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custom hotplug scripts; and when doing files or physical devices, to
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do the duplicate checking inside of libxl instead. The rationale for
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doing this in block scripts rather than in libxl isn't clear at thes
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point.
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