4. IFCVF vDPA driver

The IFCVF vDPA (vhost data path acceleration) driver provides support for the Intel FPGA 100G VF (IFCVF). IFCVF’s datapath is virtio ring compatible, it works as a HW vhost backend which can send/receive packets to/from virtio directly by DMA. Besides, it supports dirty page logging and device state report/restore, this driver enables its vDPA functionality.

4.1. IFCVF vDPA Implementation

IFCVF’s vendor ID and device ID are same as that of virtio net pci device, with its specific subsystem vendor ID and device ID. To let the device be probed by IFCVF driver, adding “vdpa=1” parameter helps to specify that this device is to be used in vDPA mode, rather than polling mode, virtio pmd will skip when it detects this message. If no this parameter specified, device will not be used as a vDPA device, and it will be driven by virtio pmd.

Different VF devices serve different virtio frontends which are in different VMs, so each VF needs to have its own DMA address translation service. During the driver probe a new container is created for this device, with this container vDPA driver can program DMA remapping table with the VM’s memory region information.

The device argument “sw-live-migration=1” will configure the driver into SW assisted live migration mode. In this mode, the driver will set up a SW relay thread when LM happens, this thread will help device to log dirty pages. Thus this mode does not require HW to implement a dirty page logging function block, but will consume some percentage of CPU resource depending on the network throughput. If no this parameter specified, driver will rely on device’s logging capability.

4.1.1. Key IFCVF vDPA driver ops

  • ifcvf_dev_config: Enable VF data path with virtio information provided by vhost lib, including IOMMU programming to enable VF DMA to VM’s memory, VFIO interrupt setup to route HW interrupt to virtio driver, create notify relay thread to translate virtio driver’s kick to a MMIO write onto HW, HW queues configuration.

    This function gets called to set up HW data path backend when virtio driver in VM gets ready.

  • ifcvf_dev_close: Revoke all the setup in ifcvf_dev_config.

    This function gets called when virtio driver stops device in VM.

4.1.2. To create a vhost port with IFC VF

  • Create a vhost socket and assign a VF’s device ID to this socket via vhost API. When QEMU vhost connection gets ready, the assigned VF will get configured automatically.

4.2. Features

Features of the IFCVF driver are:

  • Compatibility with virtio 0.95 and 1.0.

  • SW assisted vDPA live migration.

4.3. Prerequisites

  • Platform with IOMMU feature. IFC VF needs address translation service to Rx/Tx directly with virtio driver in VM.

4.4. Limitations

4.4.1. Dependency on vfio-pci

vDPA driver needs to setup VF MSIX interrupts, each queue’s interrupt vector is mapped to a callfd associated with a virtio ring. Currently only vfio-pci allows multiple interrupts, so the IFCVF driver is dependent on vfio-pci.

4.4.2. Live Migration with VIRTIO_NET_F_GUEST_ANNOUNCE

IFC VF doesn’t support RARP packet generation, virtio frontend supporting VIRTIO_NET_F_GUEST_ANNOUNCE feature can help to do that.