Linux kernel driver for Compute Engine Virtual Ethernet (gve):¶
The GVE driver binds to a single PCI device id used by the virtual Ethernet device found in some Compute Engine VMs.
The gVNIC PCI device exposes three 32-bit memory BARS: - Bar0 - Device configuration and status registers. - Bar1 - MSI-X vector table - Bar2 - IRQ, RX and TX doorbells
- The driver interacts with the device in the following ways:
- A block of MMIO registers
- See gve_register.h for more detail
- Admin Queue
- See description below
- At any time the device can be reset
- See supported interrupts below
- Transmit and Receive Queues
- See description below
All registers are MMIO and big endian.
The registers are used for initializing and configuring the device as well as querying device status in response to management interrupts.
Admin Queue (AQ)¶
The Admin Queue is a PAGE_SIZE memory block, treated as an array of AQ commands, used by the driver to issue commands to the device and set up resources.The driver and the device maintain a count of how many commands have been submitted and executed. To issue AQ commands, the driver must do the following (with proper locking):
- Copy new commands into next available slots in the AQ array
- Increment its counter by he number of new commands
- Write the counter into the GVE_ADMIN_QUEUE_DOORBELL register
- Poll the ADMIN_QUEUE_EVENT_COUNTER register until it equals the value written to the doorbell, or until a timeout.
The device will update the status field in each AQ command reported as executed through the ADMIN_QUEUE_EVENT_COUNTER register.
A device reset is triggered by writing 0x0 to the AQ PFN register. This causes the device to release all resources allocated by the driver, including the AQ itself.
The following interrupts are supported by the driver:
The management interrupt is used by the device to tell the driver to look at the GVE_DEVICE_STATUS register.
The handler for the management irq simply queues the service task in the workqueue to check the register and acks the irq.
Notification Block Interrupts¶
The notification block interrupts are used to tell the driver to poll the queues associated with that interrupt.
The handler for these irqs schedule the napi for that block to run and poll the queues.
gVNIC’s queues are composed of a descriptor ring and a buffer and are assigned to a notification block.
The descriptor rings are power-of-two-sized ring buffers consisting of fixed-size descriptors. They advance their head pointer using a __be32 doorbell located in Bar2. The tail pointers are advanced by consuming descriptors in-order and updating a __be32 counter. Both the doorbell and the counter overflow to zero.
Each queue’s buffers must be registered in advance with the device as a queue page list, and packet data can only be put in those pages.
gve maps the buffers for transmit rings into a FIFO and copies the packets into the FIFO before sending them to the NIC.
The buffers for receive rings are put into a data ring that is the same length as the descriptor ring and the head and tail pointers advance over the rings together.