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



On Tue, 21 Sep 2010, Ian Jackson wrote:
> 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).
> +

It should be made clear that for HVM guests specifying xvd* in the VM
config file means that the user is requesting a PV only disk without any
corresponding emulated disks.
As a consequence using only xvd* disks in an HVM config file is a
mistake, because grub (or any other bootloader) wouldn't be able to boot
the OS.


> + * 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

shouldn't this be xvdb2?

> +       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
> +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 use pure xvd* devices for
> +the rest.  Modern PV-on-HVM drivers will map the hd* devices to
> +/dev/xvdHDa etc.
> +

moderm PV-on-HVM drivers will map the hd* devices to /dev/xvd* etc., so
"hda1" in the config file results in /dev/xvda1 in the guest.

> +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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