3.4. Virtualized Hardware Devices

On Red Hat Enterprise Linux 7.2 and above, the host system can have up to 240 virtual CPUs (vCPUs) that can be presented to guests for use, regardless of the number of host CPUs. This is up from 160 in Red Hat Enterprise Linux 7.0.

The following core system components are emulated to provide basic system functions:

Storage devices and storage pools can use emulated drivers to attach storage devices to virtual machines. The guest uses an emulated storage driver to access the storage pool.

Note that like all virtual devices, the storage drivers are not storage devices. The drivers are used to attach a backing storage device, file or storage pool volume to a virtual machine. The backing storage device can be any supported type of storage device, file, or storage pool volume.

An emulated (Intel) HDA sound device, intel-hda, is supported in the following guest operating systems:

The following emulated graphics cards are provided.

Guests can connect to these devices with the Simple Protocol for Independent Computing Environments (SPICE) protocol or with the Virtual Network Computing (VNC) system.

The following two emulated network devices are provided:

A watchdog can be used to automatically reboot a virtual machine when the machine becomes overloaded or unresponsive.

Red Hat Enterprise Linux 7 provides the following emulated watchdog devices:

Both watchdog devices are supported on 32-bit and 64-bit AMD and Intel architectures for guest operating systems Red Hat Enterprise Linux 6.2 and above.

The paravirtualized network device is a virtual network device that provides network access to virtual machines with increased I/O performance and lower latency.

The paravirtualized block device is a high-performance virtual storage device that provides storage to virtual machines with increased I/O performance and lower latency. The paravirtualized block device is supported by the hypervisor and is attached to the virtual machine (except for floppy disk drives, which must be emulated).

The paravirtualized SCSI controller device provides a more flexible and scalable alternative to virtio-blk. A virtio-scsi guest is capable of inheriting the feature set of the target device, and can handle hundreds of devices compared to virtio-blk, which can only handle 28 devices.

virtio-scsi is fully supported for the following guest operating systems:

Guests using the Time Stamp Counter (TSC) as a clock source may suffer timing issues. KVM works around hosts that do not have a constant Time Stamp Counter by providing guests with a paravirtualized clock. Additionally, the paravirtualized clock assists with time adjustments needed after a guest runs the sleep (S3) or suspend to RAM operations.

The paravirtualized serial device is a bytestream-oriented, character stream device, and provides a simple communication interface between the host's user space and the guest's user space.

The balloon device can designate part of a virtual machine's RAM as not being used (a process known as inflating the balloon), so that the memory can be freed for the host (or for other virtual machines on that host) to use. When the virtual machine needs the memory again, the balloon can be deflated and the host can distribute the RAM back to the virtual machine.

The paravirtualized random number generator enables virtual machines to collect entropy, or randomness, directly from the host to use for encrypted data and security. Virtual machines can often be starved of entropy because typical inputs (such as hardware usage) are unavailable. Sourcing entropy can be time-consuming. virtio-rng makes this process faster by injecting entropy directly into guest virtual machines from the host.

The paravirtualized graphics card works with the QXL driver to provide an efficient way to display a virtual machine's graphics from a remote host. The QXL driver is required to use SPICE.

Virtual Function I/O (VFIO) is a new kernel driver in Red Hat Enterprise Linux 7 that provides virtual machines with high performance access to physical hardware.

VFIO attaches PCI devices on the host system directly to virtual machines, providing guests with exclusive access to PCI devices for a range of tasks. This enables PCI devices to appear and behave as if they were physically attached to the guest virtual machine.

VFIO improves on previous PCI device assignment architecture by moving device assignment out of the KVM hypervisor, and enforcing device isolation at the kernel level. VFIO offers better security and is compatible with secure boot. It is the default device assignment mechanism in Red Hat Enterprise Linux 7.

VFIO increases the number of assigned devices to 32 in Red Hat Enterprise Linux 7, up from a maximum 8 devices in Red Hat Enterprise Linux 6. VFIO also supports assignment of NVIDIA GPUs.

The KVM hypervisor supports attaching USB, PCI, and SCSI devices on the host system to virtual machines. USB, PCI, and SCSI device assignment makes it possible for the devices to appear and behave as if they were physically attached to the virtual machine. Thus, it provides guests with exclusive access to these devices for a variety of tasks.

SR-IOV (Single Root I/O Virtualization) is a PCI Express (PCI-e) standard that extends a single physical PCI function to share its PCI resources as separate virtual functions (VFs). Each function can be used by a different virtual machine via PCI device assignment.

An SR-IOV-capable PCI-e device provides a Single Root function (for example, a single Ethernet port) and presents multiple, separate virtual devices as unique PCI device functions. Each virtual device may have its own unique PCI configuration space, memory-mapped registers, and individual MSI-based interrupts.

N_Port ID Virtualization (NPIV) is a functionality available with some Fibre Channel devices. NPIV shares a single physical N_Port as multiple N_Port IDs. NPIV provides similar functionality for Fibre Channel Host Bus Adapters (HBAs) that SR-IOV provides for PCIe interfaces. With NPIV, virtual machines can be provided with a virtual Fibre Channel initiator to Storage Area Networks (SANs).

NPIV can provide high density virtualized environments with enterprise-level storage solutions.

For more information on NPIV, see the vHBA-based storage pools using SCSI devices.