[libvirt] [PATCH 1/2] qemu: add vhost-scsi-pci definitions

[Daniel P. Berrange]

On Thu, Jul 24, 2014 at 08:25:59AM +0200, Paolo Bonzini wrote:
> Il 24/07/2014 05:24, Nicholas A. Bellinger ha scritto:
> > From: Mike Perez <thingee at gmail.com>
> > 
> > This patch adds the necessary definitions to support vhost-scsi-pci and
> > VIR_DOMAIN_CONTROLLER_MODEL_SCSI_VHOST_SCSI in QEMU >= v1.5.x code.
> > 
> > This includes:
> > 
> >   - Add QEMU_CAPS_VHOST_SCSI
> >   - Add virDomainControllerDef->wwpn pointer for the vhost-scsi wwpn.
> >   - Setting the new model type in qemuSetSCSIControllerModel()
> >   - Invoking virDomainPCIAddressReserveAddr() for vhost-scsi-pci in
> >     qemuAssignDevicePCISlots()
> >   - Add the 'wwpn=' device parameter in qemuBuildControllerDevStr()
> >   - Add vhost-scsi-pci to >= v1.5.x qemucapabilitiesdata
> >   - Add xml2argv and xml2xml tests for cmd_per_lun + max_sectors +
> >     num_queues + wwpn.
> > 
> > Signed-off-by: Mike Perez <thingee at gmail.com>
> > Signed-off-by: Nicholas Bellinger <nab at linux-iscsi.org>
> 
> Configuration still has to go through configfs; I'm not sure how using <disk>
> and vhost-scsi together makes sense:
> 
> +    <disk type='block' device='disk'>
> +      <source dev='/dev/HostVG/QEMUGuest1'/>
> +      <target dev='sdb' bus='scsi'/>
> +      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
> +    </disk>
> +    <controller type='usb' index='0'/>
> +    <controller type='scsi' index='0' model='vhost-scsi'>
> +      <driver wwpn='naa.60014050a13df4f2'/>
> +    </controller>
> 
> ...
> 
> +-device vhost-scsi-pci,id=scsi0,wwpn=naa.60014050a13df4f2 \
> +-usb \
> +-drive file=/dev/HostVG/QEMUGuest1,if=none,id=drive-scsi0-0-0-0 \
> +-device scsi-disk,bus=scsi0.0,channel=0,scsi-id=0,lun=0,\
> +drive=drive-scsi0-0-0-0,id=scsi0-0-0-0 \
> 
> and I'm not sure even how it can work since vhost-scsi-pci doesn't expose
> a QEMU SCSI bus.

Indeed, it doesn't seem to actually work. I have a vm where I'm
using virtio-scsi, so I changed the model to vhost-scsi and added
a random wwpn string (unclear if there's any rules for generating
these wwpn strings?).

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/data1.img'/>
      <target dev='sda' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/data2.img'/>
      <target dev='sda' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/data3.img'/>
      <target dev='sda' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='2'/>
    </disk>
    <controller type='scsi' index='0' model='vhost-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
      <driver wwpn='naa.60014050a13df4f2'/>
    </controller>

When I try to start the guest it fails to find the SCSI controller

  qemu-system-x86_64: -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,\
  lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0: Bus 'scsi0.0' not found

A further problem is that the QEMU process itself wants access to the
/dev/vhost-scsi device, which it cannot have unless running as root,
which is not something people will do in production. It also requires
us to update the cgroups device ACL to allow access to the device and
update the SELinux policy to allow access. It is unclear to me if it
is safe to allow untrusted processes access to /dev/vhost-scsi device.
With vhost-net, libvirt itself opens the /dev/vhost-net device and
passes the pre-opened file descriptors down to QEMU, so the untrusted
processes never access the device directly, which I think is a better
model that we use quite alot in QEMU.

Regards,
Daniel
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