Hello,

 

I am currently using the code listed below for one of my drives. How can I code in the another optical drive?

 

<controller type='scsi' index='0' model='virtio-scsi'/> 
<hostdev mode='subsystem' type='scsi'>
  <source>
  <adapter name='scsi_host5'/>
  <address type='scsi' bus='0' target='0' unit='0'/>
  </source>
  <readonly/> 
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
</hostdev>

 

Thanks,

Guru

Edited April 18, 20233 yr by Greatestguru

Make sure the first part has the model equalling 'virtio-scsi'. Doing so will allow the vm to recognize the optical drives and install drivers.

 

...
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
    </controller>
    <controller type='scsi' index='1' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
    </controller>
...
    <hostdev mode='subsystem' type='scsi' managed='no'>
      <source>
        <adapter name='scsi_host2'/>
        <address bus='0' target='0' unit='0'/>
      </source>
      <readonly/>
      <address type='drive' controller='1' bus='0' target='0' unit='0'/>
    </hostdev>
    <hostdev mode='subsystem' type='scsi' managed='no'>
      <source>
        <adapter name='scsi_host6'/>
        <address bus='0' target='0' unit='0'/>
      </source>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </hostdev>
...

 

Edited June 1, 20233 yr by Greatestguru

Passing Multiple Physical Optical Drives to a Windows VM Using SCSI (libvirt / KVM)

This guide explains:

• How the original XML configuration worked with 2 optical drives

• How it was extended to support 4 physical optical drives

• Why each part of the configuration matters

The setup uses SCSI passthrough and virtio-scsi controllers.

---

1. Adding SCSI Controllers to the VM

First, the VM needs SCSI controllers to attach devices to.
In this configuration, two virtio-scsi controllers are used:

<controller type='scsi' index='0' model='virtio-scsi'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
</controller>
<controller type='scsi' index='1' model='virtio-scsi'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
</controller>

What this does:

• Creates two independent SCSI controllers inside the guest

• Uses virtio-scsi for good performance and flexibility

• Pins each controller to a fixed PCI slot so device ordering remains stable

At this point, Windows sees two empty SCSI controllers.

---

2. How a Physical Optical Drive Is Passed Through

Each physical drive is passed using a <hostdev> block with type='scsi'.

Example of a single optical drive:

<hostdev mode='subsystem' type='scsi' managed='no'>
  <source>
    <adapter name='scsi_host6'/>
    <address bus='0' target='0' unit='0'/>
  </source>
  <readonly/>
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
</hostdev>

This block has two important parts.

---

2.1 Host-side device selection

<source>
  <adapter name='scsi_host6'/>
  <address bus='0' target='0' unit='0'/>
</source>

Meaning:

• scsi_host6 identifies the SCSI controller on the Linux host

• bus / target / unit identifies the exact physical device attached to it

This tells libvirt which real optical drive to use.

---

2.2 Guest-side placement

<address type='drive' controller='0' bus='0' target='0' unit='0'/>

Meaning:

• Attaches the drive to SCSI controller 0 inside the VM

• Assigns a unique SCSI address in the guest

Every drive inside the VM must have a unique
(controller, bus, target, unit) combination.

---

2.3 Read-only flag

<readonly/>

This marks the device as read-only in the guest, which matches optical drive behavior and prevents accidental writes.

---

3. Why the Original Setup Worked for 2 Drives

• Two virtio-scsi controllers existed

• Each optical drive:

  • Used a different host SCSI adapter

  • Was attached to a different guest controller

• No guest SCSI address conflicts existed

---

4. Identifying Additional Optical Drives on the Host

To add more drives, first list them on the Linux host:

lsscsi

Example output:

[12:0:1:0]  cd/dvd  /dev/sr0
[12:0:4:0]  cd/dvd  /dev/sr1
[12:0:5:0]  cd/dvd  /dev/sr2
[12:0:6:0]  cd/dvd  /dev/sr3

Format:

[host : channel : target : lun]

From this output:

• Host adapter = 12 → scsi_host12

• Channel = 0

• Targets = 1, 4, 5, 6

• LUN = 0

---

5. Scaling from 2 Drives to 4 Drives

Instead of adding more controllers, we reused the existing two and placed two drives on each controller.

Guest-side layout:

• Controller 0:

  • unit 0

  • unit 1

• Controller 1:

  • unit 0

  • unit 1

This avoids address collisions and keeps the configuration clean.

---

6. Final Working Configuration (4 Drives)

<controller type='scsi' index='0' model='virtio-scsi'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
</controller>
<controller type='scsi' index='1' model='virtio-scsi'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
</controller>

<hostdev mode='subsystem' type='scsi' managed='no'>
  <source>
    <adapter name='scsi_host12'/>
    <address bus='0' target='1' unit='0'/>
  </source>
  <readonly/>
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
</hostdev>

<hostdev mode='subsystem' type='scsi' managed='no'>
  <source>
    <adapter name='scsi_host12'/>
    <address bus='0' target='4' unit='0'/>
  </source>
  <readonly/>
  <address type='drive' controller='0' bus='0' target='0' unit='1'/>
</hostdev>

<hostdev mode='subsystem' type='scsi' managed='no'>
  <source>
    <adapter name='scsi_host12'/>
    <address bus='0' target='5' unit='0'/>
  </source>
  <readonly/>
  <address type='drive' controller='1' bus='0' target='0' unit='0'/>
</hostdev>

<hostdev mode='subsystem' type='scsi' managed='no'>
  <source>
    <adapter name='scsi_host12'/>
    <address bus='0' target='6' unit='0'/>
  </source>
  <readonly/>
  <address type='drive' controller='1' bus='0' target='0' unit='1'/>
</hostdev>

---

7. Key Takeaways

• Each physical drive requires its own <hostdev> block

• Host SCSI addresses must match lsscsi output exactly

• Guest SCSI addresses must be unique

• Reusing controllers with different unit values scales cleanly

• This approach works reliably for multiple optical drives in Windows VMs