Virtual I/O Internals
The following document will attempt to detail the internals of a Virtual Function IO (VFIO) driven Mediated Device (Mdev).
| 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
VFIO file descriptor
VFIO devices are mapped as file offsets to represent the IO device.
In the case of a RPC Mode this structure is emulated whereas in SR-IOV Mode the structure is mapped to a real PCI resource.
| 00:00.0 VGA compatible controller |
|---|
| Region 0 Bar0 (starts at offset 0) |
| Region 1 Bar1 |
| Region 2 Bar2 |
| 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.
| <- 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) | ... |
RPC Mode
Instruction Execution
RPC Mode moves instruction information across a virtual function interface (VF) using Remote Procedure Calls generally by way of soft interrupt (IOCTLs). 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.
RPC Mode Requirements:
Device SR-IOV support.
HPA<->GPA Boundary Enforcement.