Virtual I/O Internals

The following document will attempt to detail the internals of a Virtual Function IO (VFIO) driven Mediated Device (Mdev).

Comparison of Approaches

RPC Mode	SR-IOV Mode
Host requires insight about guest of workload.	Host ignorance of guest workload.
Error reporting.	No guest driver error reporting.
In depth dynamic monitoring.	Basic dynamic monitoring.
Software defined MMU guest separation.	Firmware defined MMU guest separation.
Requires deferred instructions to be supported by host software (support libraries).	Guest is ignorant of host supported software such as support libraries.

Both Modes

This section will cover concepts which apply both to RPC Mode and SR-IOV Mode.

Binding VFIO devices

Figure 1: VFIO group nodes are unit of ownership that VFIO uses.

Figure 2: IOCTL(GROUP, VFIO_GROUP_SET_CONTAINER, &CONTAINER) places the VFIO Group inside the VFIO Container.

Figure 3: Using interrupts IOCTL(CONTAINER, VFIO_IOMMU_MAP_DMA, &MAP), IOCTL(CONTAINER, VFIO_IOMMU_UNMAP_DMA, &UNMAP) to map memory and pin pages.

Figure 4: Using interrupt IOCTL(GROUP2, VFIO_GROUP_GET_FD, "0000:01:00.0") to obtain the VFIO Group file descriptor.

Figure 1: Binding devices to the vfio-pci driver results in VFIO group nodes.

Opening the file "/dev/vfio/vfio" creates a VFIO Container.

Figure 2: The interrupt routine IOCTL(GROUP, VFIO_GROUP_SET_CONTAINER, &CONTAINER) places the VFIO group inside the VFIO container.

Programming the IOMMU

When this has been done IOCTL(CONTAINER, VFIO_SET_IOMMU, VFIO_TYPE1_IOMMU) can then be used to set an IOMMU type for the container which places it in a user interact-able state.

Once this IOMMU type state has been set and the VFIO container has been made interact-able additional VFIO groups may be added to the container without requiring that the group's IOMMU type be set again as newly added groups automatically inherit the container's IOMMU context.

VFIO Memory Mapped IO

Figure 3: Once the VFIO Groups have been placed inside the VFIO container and the IOMMU type has been set the user may then map and unmap which will automatically inserts Memory Mapped IO (MMIO) entries into the IOMMU as well as pin/unpin pages as necessary. This can be accomplished using IOCTL(CONTAINER, VFIO_IOMMU_MAP_DMA, &MAP) for map/pin and IOCTL(CONTAINER, VFIO_IOMMU_UNMAP_DMA, &UNMAP) for unmap/unpin.

Getting the VFIO Group File Descriptor

Figure 4: Once the device has been bound to a VFIO driver, set in a VFIO container, the VFIO container has it's IOMMU type set, and a memory map/page pin of the VFIO device has been completed a file descriptor can then be obtained for the device. This file descriptor can be used for interrupts (ioctls), to probe for information about the BAR regions, and configure the IRQs.

VFIO device file descriptor

VFIO device file descriptors are divided into regions and each region is mapped into a device resource. Region count and info (file offset, allowable access, ect..) can be discovered through interrupt (IOCTL). Each file descriptor region corresponding to a PCI resource is represented as a file offset.

In the case of RPC Mode this structure is emulated whereas in SR-IOV Mode the structure is mapped to a real PCI resource.

BAR regions in a VGA PCI device.

00:00.0 VGA compatible controller
Region 0 Bar0 (starts at offset 0)
Region 1 Bar1 (MSI)
Region 2 Bar2 (MSIX)
Region 3 Bar3
Region 4 Bar4
Region 5 Bar5 (IO port space)
Expansion ROM

Below is what the file offsets looks like internally for each BAR region starting from address 0 and growing with the addition of former regions as you progress through the file.

VFIO representation of PCI BAR regions offsets.

<- File Offset ->
0 -> A	A -> (A+B)	(A+B) -> (A+B+C)	(A+B+C) -> (A+B+C+D)	...
Region 0 (size A)	Region 1 (size B)	Region 2 (size C)	Region 3 (size D)	...

VFIO Interrupts

Guests communicate with the host via VFIO Interrupt Requests (IRQs). These are sent via an irqfd (IRQ File Descriptor). Similarly, the host receives these interrupts via eventfd (Event File Descriptor). The resulting data can be returned via a callback.

IRQs

Device properties discovered via interrupt (IOCTL).

Get Device Info

VFIO_DEVICE_GET_INFO
struct vfio_device_info
	argz
	flags
		VFIO_DEVICE_FLAGS_PCI
		VFIO_DEVICE_FLAGS_PLATFORM
		VFIO_DEVICE_FLAGS_RESET
	num_irqs
	num_regions

The IRQ VFIO_DEVICE_GET_INFO can provide information to distinguish between PCI and platform devices as well as the number of regions and IRQs for a particular device.

Get Region Info

VFIO_DEVICE_GET_REGION_INFO
struct vfio_region_info
	argz
	cap_offset
	flags
		VFIO_REGION_INFO_FLAG_CAPS
		VFIO_REGION_INFO_FLAG_MMAP
		VFIO_REGION_INFO_FLAG_READ
		VFIO_REGION_INFO_FLAG_WRITE
	index
	offset
	size

Once the interrupt user knows the number of regions within a VFIO device they can use IRQ VFIO_DEVICE_GET_REGION_INFO to probe each region for additional information. This interrupt will return information such as if it can be read from or written to, if the device supports MMAP, as well as what the offset and size of the region is within the VFIO file descriptor.

Get IRQ Info

VFIO_DEVICE_GET_IRQ_INFO
struct vfio_irq_info
	argz
	count
	flags
		VFIO_IRQ_INFO_AUTOMASKED
		VFIO_IRQ_INFO_EVENTFD
		VFIO_IRQ_INFO_MASKABLE
		VFIO_IRQ_INFO_NORESIZE
	index

VFIO_IRQ_INFO_AUTOMASKED is used to mask interrupts when they occur to protect the host.

Set IRQs

VFIO_DEVICE_SET_IRQS
struct vfio_irq_set
	argz
	count
	data[]
	flags
		VFIO_IRQ_SET_ACTION_MASK
		VFIO_IRQ_SET_ACTION_TRIGGER
		VFIO_IRQ_SET_ACTION_UNMASK
		VFIO_IRQ_SET_DATA_BOOL
		VFIO_IRQ_SET_DATA_EVENTFD
		VFIO_IRQ_SET_DATA_NONE
	index
	start

RPC Mode

Instruction Execution

A simple diagram of signalling from host to guest (via irqfd) & guest to host (via ioeventfd) from blog.allenx

RPC Mode moves instruction information across a virtual function (VF) device using Remote Procedure Calls generally by way of soft interrupt (IOCTLs). These signals may be passed over file descriptors such as irqfd and ioeventfd. The Interrupt Request File Descriptor (irqfd) may be used to signal from the host into the guest whereas the I/O Event File Descriptor (ioeventfd) may be used to signal from the guest into the host. Guest GPU instructions passed from the guest as Remote Procedure Calls are Just-in-time recompiled on the host for execution by a device driver.

IRQ remapping

Interrupt Requests (IRQs) must be remapped (trapped for virtualized execution) to protect the host from sensitive instructions which may affect global memory state.

Memory Management

Region Passthrough

Guests may be presented with emulated memory regions which use indirect emulated communication requiring a VM-exit (slow) or instead the guest may be presented with passthrough memory regions which use direct communication requiring no VM-exit (fast).

EPT Page Violations

Guest Memory Mapped IO (MMIO) tripped Extended Page Table (EPT) violations which are trapped by the host MMU. KVM services EPT violations and forwards to QEMU VFIO PCI driver. QEMU then converts the request from KVM to R/W access to the Mdev File Descriptor (FD). Reads and writes are then handled by the host GPU device driver via mediated callbacks (CBs) and VFIO-mdev.

Scheduling

Scheduling is handled by the host mdev driver.

RPC Mode Requirements:

Sensitive Instruction List.

Instruction Shim/Binary Translator.

HPA<->GPA Boundary Enforcement.

SR-IOV Mode

Instruction Execution

SR-IOV Mode involves the communication of instructions from a virtual function (VF) through direct communication to the PCI BAR.

Memory Management

Guests are presenting with passthrough memory regions by the device firmware.

Scheduling

Scheduling may be handled by the host mdev driver and/or the device firmware.

SR-IOV Mode Requirements:

Device SR-IOV support.

HPA<->GPA Boundary Enforcement.

Talks & Reading Material

Auxiliary Bus

IOCTL

eventfd - root/virt/kvm/eventfd.c

(kernel diff:: KVM: irqfd)

(kernel diff:: KVM: add ioeventfd support)

KVM irqfd & ioeventfd

VFIO - Virtual Function I/O - root/virt/kvm/vfio.c

VFIO Mediated Devices

[2016] An Introduction to PCI Device Assignment with VFIO by Alex Williamson

Intel GVT-g: From Production to Upstream - Zhi Wang, Intel

Hardware-Assisted Mediated Pass-Through with VFIO by Kevin Tian

[2016] vGPU on KVM - A VFIO Based Framework by Neo Jia & Kirti Wankhede

[2017] Generic Buffer Sharing Mechanism for Mediated Devices by Tina Zhang