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12.5. PCI Device Restrictions

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 PCIe 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/libvirt/qemu.conf 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