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[Xen-devel] [RFC, PATCH] docs: Block numbering and naming specification

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Subject: [Xen-devel] [RFC, PATCH] docs: Block numbering and naming specification
From: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Date: Tue, 21 Sep 2010 16:24:10 +0100
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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This document describes the vbd device numbering and naming.  I've
posted versions of it before.  It should be in docs/misc, so here is a
patch to add it.

This is currently an RFC because the section near the bottom about the
behaviour of Linux guests needs to be checked for accuracy.  In
particular, it would be good for Stefano or Jeremy to confirm the
behaviour of current pvops kernels.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>

diff -r 77a3da957017 docs/misc/block-numbering-naming.txt
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/misc/block-numbering-naming.txt      Tue Sep 21 16:20:53 2010 +0100
@@ -0,0 +1,124 @@
+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
+   thee 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     xvda2    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 scheme we therefore 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.
+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 provided essentially 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
+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
+       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 use pure xvd* devices for
+the rest.  Modern PV-on-HVM drivers will map the hd* devices to
+/dev/xvdHDa etc.
+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.


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