virtio-scsi is an advanced storage stack for KVM. It replaces the former virtio-blk stack for SCSI devices pass-through. It has several advantages over virtio-blk:

qemu-system-x86_64 [...] \
-device virtio-scsi-pci,id=scsi

The vhost-net module is used to accelerate KVM's paravirtualized network drivers. It provides better latency and greater throughput for network. Use the vhost-net driver by starting the guest with the following example command line:

qemu-system-x86_64 [...] \
-netdev tap,id=guest0,vhost=on,script=no \
-net nic,model=virtio,netdev=guest0,macaddr=00:16:35:AF:94:4B

Note that guest0 is an identification string of the vhost-driven device.

As the number of virtual CPUs increases in VM Guests, QEMU offers a way of improving the network performance using multiqueue. Multiqueue virtio-net scales the network performance by allowing VM Guest virtual CPUs to transfer packets in parallel. Multiqueue support is required on both VM Host Server and VM Guest side.

Tip: Performance Benefit

The Multiqueue virtio-net solution is most beneficial in the following cases:

• Network traffic packets are large.

• VM Guest has more connections active at the same time, mainly between the guest systems, or between the guest and the host, or between the guest and an external system.

• The number of active queues is equal to the number of virtual CPUs in the VM Guest.

Note

While multiqueue virtio-net increases the total network throughput, it increases CPU consumption as it uses of the virtual CPU's power.

The resulting qemu-system-ARCH command line will look similar to the following example:

qemu-system-x86_64 [...] -netdev tap,id=guest0,queues=4,vhost=on \
-device virtio-net-pci,netdev=guest0,mq=on,vectors=10

Note that the id of the network device (guest0 ) needs to be identical for both options.

Inside the running VM Guest, specify the following command as root:

ethtool -L eth0 combined 8

Now the guest system networking uses the multiqueue support from the qemu-system-ARCH hypervisor.

Directly assigning a PCI device to a VM Guest (PCI pass-through) avoids performance issues caused by avoiding any emulation in performance critical paths. VFIO replaces the traditional KVM PCI Pass-Through device assignment. Prerequisite for this feature is a VM Host Server configuration described in Important: Requirements for VFIO and SR-IOV.

To be able to assign a PCI device via VFIO to a VM Guest, you need to find out which IOMMU Group it belongs to. The IOMMU (input/output memory management unit that connects a direct memory access-capable I/O bus to the main memory) API supports the notion of groups. A group is a set of devices that can be isolated from all other devices in the system. Groups are therefore the unit of ownership used by VFIO.

Important: No Hot-plugging

As of openSUSE Leap 42.1 hot-plugging of PCI devices passed to a VM Guest via VFIO is not supported.

You can find more detailed information on the VFIO driver in the /usr/src/linux/Documentation/vfio.txt file (package kernel-source has to be installed).

To assign an existing host USB device to a VM Guest, you need to find out its host bus and device ID.

tux@vmhost:~> lsusb
[...]
Bus 002 Device 005: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750
[...]

In the above example, we want to assign a USB stick connected to the host's USB bus number 2 with device number 5. Now run the VM Guest with the following additional options:

qemu-system-x86_64 [...] -usb -device usb-host,hostbus=2,hostaddr=5

After the guest is booted, check that the assigned USB device is present on it.

tux@vmguest:~> lsusb
[...]
Bus 001 Device 002: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750
[...]

Note

The guest operating system must take care of mounting the assigned USB device so that it is accessible for the user.

PCI Pass-Through is a technique to give your VM Guest exclusive access to a VM Host Server PCI device. Prerequisite for this feature is a VM Host Server configuration described in Important: Requirements for VFIO and SR-IOV. Assigning graphics cards is not supported by SUSE.

Important: PCI Pass-Through is deprecated

KVM PCI Pass-Through has been replaced by VFIO (see VFIO for more information). Although it is still supported by SUSE, it is recommended to use VFIO for assigning PCI devices as described in Section 27.3.4, “VFIO: Secure Direct Access to Devices”. vt.io.vfio

Note: IRQ Sharing

If the PCI device shares IRQ with other devices, it cannot be assigned to a VM Guest.

KVM also supports PCI device hot-plugging to a VM Guest. To achieve this, you need to switch to a QEMU monitor (see Chapter 30, Virtual Machine Administration Using QEMU Monitor for more information) and issue the following commands:

VM Guests usually run in a separate computing space—they are provided their own memory range, dedicated CPUs, and file system space. The ability to share parts of the VM Host Server's file system makes the virtualization environment more flexible by simplifying mutual data exchange. Network file systems, such as CIFS and NFS, have been the traditional way of sharing directories. But as they are not specifically designed for virtualization purposes, they suffer from major performance and feature issues.

KVM introduces a new optimized method called VirtFS (sometimes called “file system pass-through”). VirtFS uses a paravirtual file system driver, which avoids converting the guest application file system operations into block device operations, and then again into host file system operations.

You typically use VirtFS for the following situations:

qemu-system-x86_64 [...] \
-fsdev local,id=exp11,path=/tmp/2,security_model=mapped3 \
-device virtio-9p-pci,fsdev=exp14,mount_tag=v_tmp5

sudo mount -t 9p -o trans=virtio v_tmp /mnt

Kernel SamePage Merging (KSM) is a Linux Kernel feature that merges identical memory pages from multiple running processes into one memory region. Because KVM guests run as processes under Linux, KSM provides the memory overcommit feature to hypervisors for more efficient use of memory. Therefore, if you need to run multiple virtual machines on a host with limited memory, KSM may be helpful to you.

KSM stores its status information in the files under the /sys/kernel/mm/ksm directory:

tux > ls -1 /sys/kernel/mm/ksm
full_scans
merge_across_nodes
pages_shared
pages_sharing
pages_to_scan
pages_unshared
pages_volatile
run
sleep_millisecs

For more information on the meaning of the /sys/kernel/mm/ksm/* files, see /usr/src/linux/Documentation/vm/ksm.txt (package kernel-source).

To use KSM, do the following.