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Xen guest interface
-------------------
A Xen guest can be provided with block devices. These are always
provided as Xen VBDs; for HVM guests they may also be provided as
emulated IDE or SCSI disks.
The abstract interface involves specifying, for each block device:
* Nominal disk type: Xen virtual disk (aka xvd*, the default); SCSI
(sd*); IDE (hd*).
For HVM guests, each whole-disk hd* and and sd* device is made
available _both_ via emulated IDE resp. SCSI controller, _and_ as a
Xen VBD. The HVM guest is entitled to assume that the IDE or SCSI
disks available via the emulated IDE controller target the same
underlying devices as the corresponding Xen VBD (ie, multipath).
For PV guests every device is made available to the guest only as a
Xen VBD. For these domains the type is advisory, for use by the
guest's device naming scheme.
The Xen interface does not specify what name a device should have
in the guest (nor what major/minor device number it should have in
the guest, if the guest has such a concept).
* Disk number, which is a nonnegative integer,
conventionally starting at 0 for the first disk.
* Partition number, which is a nonnegative integer where by
convention partition 0 indicates the "whole disk".
Normally for any disk _either_ partition 0 should be supplied in
which case the guest is expected to treat it as they would a native
whole disk (for example by putting or expecting a partition table
or disk label on it);
_Or_ only non-0 partitions should be supplied in which case the
guest should expect storage management to be done by the host and
treat each vbd as it would a partition or slice or LVM volume (for
example by putting or expecting a filesystem on it).
Non-whole disk devices cannot be passed through to HVM guests via
the emulated IDE or SCSI controllers.
Configuration file syntax
-------------------------
The config file syntaxes are, for example
d0 d0p0 xvda Xen virtual disk 0 partition 0 (whole disk)
d1p2 xvdb2 Xen virtual disk 1 partition 2
d536p37 xvdtq37 Xen virtual disk 536 partition 37
sdb3 SCSI disk 1 partition 3
hdc2 IDE disk 2 partition 2
The d*p* syntax is not supported by xm/xend.
To cope with guests which predate this specification we preserve the
existing facility to specify the xenstore numerical value directly by
putting a single number (hex, decimal or octal) in the domain config
file instead of the disk identifier; this number is written directly
to xenstore (after conversion to the canonical decimal format).
Concrete encoding in the VBD interface (in xenstore)
----------------------------------------------------
The information above is encoded in the concrete interface as an
integer (in a canonical decimal format in xenstore), whose value
encodes the information above as follows:
1 << 28 | disk << 8 | partition xvd, disks or partitions 16 onwards
202 << 8 | disk << 4 | partition xvd, disks and partitions up to 15
8 << 8 | disk << 4 | partition sd, disks and partitions up to 15
3 << 8 | disk << 6 | partition hd, disks 0..1, partitions 0..63
22 << 8 | (disk-2) << 6 | partition hd, disks 2..3, partitions 0..63
2 << 28 onwards reserved for future use
other values less than 1 << 28 deprecated / reserved
The 1<<28 format handles disks up to (1<<20)-1 and partitions up to
255. It will be used only where the 202<<8 format does not have
enough bits.
Guests MAY support any subset of the formats above except that if they
support 1<<28 they MUST also support 202<<8. PV-on-HVM drivers MUST
support at least one of 3<<8 or 8<<8; 3<<8 is recommended.
Some software has used or understood Linux-specific encodings for SCSI
disks beyond disk 15 partition 15, and IDE disks beyond disk 3
partition 63. These vbds, and the corresponding encoded integers, are
deprecated.
Guests SHOULD ignore numbers that they do not understand or
recognise. They SHOULD check supplied numbers for validity.
Notes on Linux as a guest
-------------------------
Very old Linux guests (PV and PV-on-HVM) are able to "steal" the
device numbers and names normally used by the IDE and SCSI
controllers, so that writing "hda1" in the config file results in
/dev/hda1 in the guest. These systems interpret the xenstore integer
as
major << 8 | minor
where major and minor are the Linux-specific device numbers. Some old
configurations may depend on deprecated high-numbered SCSI and IDE
disks. This does not work in recent versions of Linux.
So for Linux PV guests, users are recommended to supply xvd* devices
only. Modern PV drivers will map these to identically-named devices
in the guest.
For Linux HVM guests using PV-on-HVM drivers, users are recommended to
supply as few hd* devices as possible, and for the rest of the disks,
to use pure xvd* devices starting at xvde. Modern PV-on-HVM drivers
will map provided hd* devices to the corresponding /dev/xvd* (for
example, hda is presented also as /dev/xvda).
Some Linux HVM guests with broken PV-on-HVM drivers do not cope
properly if both hda and hdc are supplied, nor with both hda and xvda,
because they directly map the bottom 8 bits of the xenstore integer
directly to the Linux guest's device number and throw away the rest;
they can crash due to minor number clashes. With these guests, the
workaround is not to supply problematic combinations of devices.
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