How to assign devices with VT-d in KVM
- In order to assign devices in KVM, you'll need a system which supports VT-d, not to be confused with the VT-x support of your CPU. VT-d needs to be supported by both your motherboard chipset and your CPU.
- If you are in doubt whether your motherboard or CPU supports VT-d, the Xen VT-d wiki has some information about VT-d enabled chipsets, motherboards, and CPUs:
- If your hardware does not have an IOMMU (known as "Intel VT-d" on Intel-based machines and "AMD I/O Virtualization Technology" on AMD-based machines), you will not be able to assign devices in KVM.
- Assigning graphics cards is not officially supported at the moment, but there has been some success passing through a secondary Radeon HD 5850 as a VM's secondary display.
Assigning device to guest
1. Modify the kernel config
- make menuconfig
- set "Bus options (PCI etc.)" -> "Support for DMA Remapping Devices" to "*"
- set "Bus options (PCI etc.)" -> "Enable DMA Remapping Devices" to "*"
- set "Bus options (PCI etc.)" -> "PCI Stub driver" to "*"
- optional setting:
set "Bus options (PCI etc.)" -> "Support for Interrupt Remapping" to "*"
2. Build the kernel
- make modules_install
- make install
3. Reboot and verify that your system has IOMMU support
- AMD Machine
- dmesg | grep AMD-Vi
... AMD-Vi: Enabling IOMMU at 0000:00:00.2 cap 0x40 AMD-Vi: Lazy IO/TLB flushing enabled AMD-Vi: Initialized for Passthrough Mode ...
- Intel Machine
- dmesg | grep -e DMAR -e IOMMU
... DMAR:DRHD base: 0x000000feb03000 flags: 0x0 IOMMU feb03000: ver 1:0 cap c9008020e30260 ecap 1000 ...
- If you get no output you'll need to fix this before moving on. Check if your hardware supports VT-d and check that it has been enabled in BIOS.
NOTE: If you still get an error "No IOMMU found." Check dmesg for errors suggesting your BIOS is broken. Another possible reason: CONFIG_DMAR_DEFAULT_ON is not set. In that case, pass "intel_iommu=on" as kernel parameter to enable it. AMD uses different kernel parameter than Intel, on AMD you will need to pass "iommu=pt iommu=1".
4. Unbind the device from the host kernel driver (Example: PCI device 01:00.0)
- Load the PCI Stub Driver if it is compiled as a module
- lspci -n
- locate the entry for device 01:00.0 and note down the vendor & device ID 8086:10b9
... 01:00.0 0200: 8086:10b9 (rev 06) ...
- echo "8086 10b9" > /sys/bus/pci/drivers/pci-stub/new_id
- echo 0000:01:00.0 > /sys/bus/pci/devices/0000:01:00.0/driver/unbind
- echo 0000:01:00.0 > /sys/bus/pci/drivers/pci-stub/bind
5. Load the KVM modules
- modprobe kvm
- modprobe kvm-intel
6. Assign the device
- /usr/local/bin/qemu-system-x86_64 -m 512 -boot c -net none -hda /root/ia32e_rhel5u1.img -device pci-assign,host=01:00.0
VT-d device hotplug
KVM also supports hotplug devices with VT-d to guest. In guest command interface (you can press Ctrl+Alt+2 to enter it), you can use following command to hot add/remove devices to/from guest:
- hot add:
- hot remove:
- VT-d spec specifies that all conventional PCI devices behind a PCIe-to PCI/PCI-X bridge or conventional PCI bridge can only be collectively assigned to the same guest. PCIe devices do not have this restriction.
- If the device doesn't support MSI, and it shares IRQ with other devices, then it cannot be assigned due to host irq sharing for assigned devices is not supported. You will get warning message when you assign it. Notice this also apply to the devices which only support MSI-X.