[libvirt] virtio-scsi support proposal, v2

[Osier Yang]

On 2011年12月23日 16:36, Paolo Bonzini wrote:
> 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").

IMHO "device=lun" is the right way to go here, per we want the
the device is exposed to guest as a LUN but not a normal disk.
But it seems for Laine's patch, it's also right to use "type=lun",
as it tries to disable/enable SG_IO for normal disk?

> 
> 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>
>