Installation guide

Physically shared devices
5
Para-virtualized block driver
The para-virtualized block driver is a driver for all storage devices supported by the hypervisor
attached to the virtualized guest (except for floppy disk drives, which must be emulated).
The para-virtualized clock
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
para-virtualized clock.
For more information on the para-virtualized clock refer to Chapter 14, KVM guest timing
management.
The para-virtualized serial driver
The para-virtualized serial driver (virtio-serial) is a bytestream-oriented, character stream driver. The
para-virtualized serial driver provides a simple communication interface between the host's user space
and the guest's user space where networking is not available or unusable.
The balloon driver
The balloon driver allows guests to express to the hypervisor how much memory they require. The
balloon driver allows the host to efficiently allocate memory to the guest and allow free memory to be
allocated to other guests and processes.
Guests using the balloon driver can mark sections of the guest's RAM as not in use (balloon inflation).
The hypervisor can free the memory and use the memory for other host processes or other guests on
that host.
When the guest requires the freed memory again, the hypervisor can reallocate RAM to the guest
(balloon deflation).
1.4.3. Physically shared devices
Certain hardware platforms allow virtualized guests to directly access various hardware devices and
components. This process in virtualization is known as passthrough. Passthrough is known as device
assignment in some of the KVM documentation and the KVM code.
PCI passthrough
The KVM hypervisor supports attaching PCI devices on the host system to virtualized guests.
PCI passthrough allows guests to have exclusive access to PCI devices for a range of tasks. PCI
passthrough allows PCI devices to appear and behave as if they were physically attached to the guest
operating system.
Almost all PCI and PCI Express devices that support passthrough, except for graphics cards, can be
directly attached to virtualized guests with PCI passthrough.
SR-IOV
SR-IOV (Single Root I/O Virtualization) is a standard for a type of PCI passthrough which natively
shares a single device to multiple guests.
SR-IOV enables a Single Root Function (for example, a single Ethernet port), to appear as multiple,
separate, physical devices. A physical device with SR-IOV capabilities can be configured to appear