Here is a revised version of the virtio-scsi proposal. There's actually
not too much left intact from v1. :)
The main simplification is in how SCSI hosts can be addressed in a stable
manner.
SCSI controller models
======================
Existing controller models are "auto", "buslogic", "lsilogic", "lsias1068",
or "vmpvscsi". The new controller model "virtio-scsi" is added. The model
"lsilogic" is mapped to the existing "lsi" device in QEMU.
When PPC64 support will be added, another controller model "spapr-vscsi"
will be added.
Stable addressing for SCSI devices
==================================
The existing <address type='drive' ...> element will be extended as follows:
<address type='drive' controller='...'
bus='...' target='...' unit='...'/>
where controller selects the qdev parent device, while bus/target/unit
are passed as qdev properties (the QEMU names are respectively channel,
scsi-id, lun).
Libvirt should check for the QEMU "scsi-disk.channel" property. If it
is unavailable, QEMU will only support channel=lun=0 and 0<=target<=7.
LUN passthrough: block devices
==============================
A SCSI block device from the host can be attached to a domain in two
ways: as an emulated LUN with SCSI commands implemented within QEMU,
or by passing SCSI commands down to the block device. The former is
handled by the existing <disk type='file'>, <disk type='block'> and
<disk type='network'> XML syntax. The latter is not yet supported.
On the QEMU side, LUN passthrough is implemented by one of the
scsi-generic and scsi-block devices. Scsi-generic requires a /dev/sg
device name, and can be applied to any device. scsi-block is only
available in QEMU 1.0 or newer, requires a block device, can be applied
only to block devices (sd/sr) and has better performance.
To implement LUN passthrough for block device, libvirt will add a new
<disk device='lun'> attribute. When, device='lun' is passed, the device
attribute is ignored.
Example:
<disk type='block' device='lun'>
<disk name='qemu' type='raw'/>
<source dev='/dev/sda'/>
<target dev='sda' bus='scsi'>
<address type='drive' controller='...'
bus='...' target='...' unit='...'/>
</disk>
Also, virtio-blk handling will be enhanced to disable SG_IO passthrough
when <disk device='disk'>, and only enable it when <disk device='lun'>.
(I am not sure whether the 'lun' value should be for the type or device
attribute. Laine has a patch to implement it for virtio disks which
uses "type").
This syntax makes it clear what is the passed-through device, and at
the same time it makes it very easy to switch a disk between emulated
and passthrough modes. Also, a stable addressing for the source device
is provided by /dev/disk/by-id and /dev/disk/by-path.
Stable SCSI host addressing
===========================
SCSI host number in Linux is not stable. An alternative stable
addressing is required to pass a whole host or target to a guest.
One place in which this could be supported is the SCSI volume pool
syntax:
<pool type='scsi'>
<name>virtimages</name>
<source>
<adapter name='host0'/>
</source>
<target>
<path>/dev/disk/by-id</path>
</target>
</pool>
libvirt will deprecate the above form for the adapter element and
provide the following forms:
<adapter name='scsi_host0'/>
<adapter parent='pci_0000_00_1f_2' unique_id='1'/>
The existing form changes from host0 to scsi_host0, for
consistency with the naming that is used in nodedev. The new
parent/unique_id addressing uses a parent PCI device and a unique
id that Linux provides in sysfs. In order to determine the SCSI
host number, libvirt would scan all files matched by the glob pattern
/sys/bus/pci/devices/0000:00:1f.2/*/scsi_host/*/unique_id, looking for
the one that contains "1".
The unique_id can be omitted. In this case, the pool will refer
to the host with the smallest unique_id under the given device.
Furthermore, a SCSI pool can be restricted to one target using an
additional element:
<source>
<adapter name='scsi_host0'/>
<address type='scsi' bus='0' target='0'/>
</source>
(bus defaults to 0, target is mandatory).
Generic passthrough
===================
Generic device passthrough at the LUN, target or host level builds
on the extensions to SCSI addressing from the previous section.
Passing a single LUN extends the <hostdev> tag as follows:
<hostdev type='scsi'>
<source>
<adapter name='scsi_host0'/>
<address type='scsi' bus='0' target='0' unit='0'/>
</source>
<target>
<address type='scsi' controller='...'
bus='...' target='...' unit='...'/>
</target>
</hostdev>
This will map to a -drive QEMU option referring to a scsi-generic
device, and a "-device scsi-generic" option referring to the drive.
libvirt can determine the /dev/sg file to use by reading the directory
/sys/bus/scsi/devices/target*/*/scsi_generic. These devices might also
be shown in the nodedev tree, similar to block devices.
Whenever a domain should receive all devices belonging to a SCSI host,
a similar <source> item should be included within the <controller
type='scsi'> element:
<controller type='scsi' model='virtio-scsi'>
<source>
<adapter name='scsi_host0'/>
</source>
</controller>
In this case, libvirt should use scsi-block rather than scsi-generic
for block devices.
NPIV-based SCSI host passthrough
================================
In NPIV, a virtual HBA is created using "virsh nodedev-create" and passed
to the guest. Passing through a whole SCSI host is quite common when
using NPIV. As a result, it is desirable to easily address virtual HBAs
both in SCSI storage pools and in <controller type='scsi'> elements.
Here are two proposals for how to refer to NPIV adapters:
1) add persistent nodedevs via commands nodedev-define, nodedev-undefine,
nodedev-start. The persistent nodedevs have a name, and this can be
used simply with <adapter name='NAME'>.
2) Virtual adapters do have a stable address, namely its WWN. This
can be used in a third <adapter> syntax:
<source>
<adapter type='fc_host' wwpn='...' wwnn='...'/>
</source>
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