Installation guide

SCSI emulation is not supported with KVM in Red Hat Enterprise Linux.
Virt u aliz ed ID E d evices
KVM is limited to a maximum of four virtualized (emulated) IDE devices per guest virtual
machine.
PCI d evices
Red Hat Enterprise Linux 6 supports 32 PCI device slots per virtual machine, and 8 PCI
functions per device slot. This gives a theoretical maximum of 256 PCI functions per guest
when multi-function capabilities are enabled.
However, this theoretical maximum is subject to the following limitations:
Each virtual machine supports a maximum of 8 assigned device functions.
4 PCI device slots are configured with 5 emulated devices (two devices are in slot 1) by
default. However, users can explicitly remove 2 of the emulated devices that are
configured by default if the guest operating system does not require them for operation
(the video adapter device in slot 2; and the memory balloon driver device in the lowest
available slot, usually slot 3). This gives users a supported functional maximum of 30
PCI device slots per virtual machine.
The following restrictions also apply to PCI device assignment:
PCI device assignment (attaching PCI devices to virtual machines) requires host
systems to have AMD IOMMU or Intel VT-d support to enable device assignment of PCI-e
devices.
For parallel/legacy PCI, only single devices behind a PCI bridge are supported.
Multiple PCIe endpoints connected through a non-root PCIe switch require ACS support
in the PCIe bridges of the PCIe switch. To disable this restriction, edit the
/etc/l i bvi rt/q emu. co nf file and insert the line:
relaxed_acs_check=1
Red Hat Enterprise Linux 6 has limited PCI configuration space access by guest device
drivers. This limitation could cause drivers that are dependent on PCI configuration
space to fail configuration.
Red Hat Enterprise Linux 6.2 introduced interrupt remapping as a requirement for PCI
device assignment. If your platform does not provide support for interrupt remapping,
circumvent the KVM check for this support with the following command as the root user
at the command line prompt:
# echo 1 >
/sys/module/kvm/parameters/allow_unsafe_assigned_interrupts
Migrat io n rest rict io n s
Device assignment refers to physical devices that have been exposed to a virtual machine,
for the exclusive use of that virtual machine. Because device assignment uses hardware on
the specific host where the virtual machine runs, migration and save/restore are not
supported when device assignment is in use. If the guest operating system supports hot-
plugging, assigned devices can be removed prior to the migration or save/restore operation
to enable this feature.
⁠Chapt er 4 . Virt ualizat ion rest rict ions
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