Background on Linux Networking vs. Userspace Networking

Linux Networking:
- Many copies, context swithes, interrupts and queueing

Userspace networking:
- Idea is that application directly writes into network queues and reads
  from network queues
- By "device driver queues" -- we mean the application directly writes and reads
  from packets that the NIC can access
- When used with a run-to-completion scheduling model, removes context switch
  and interrupt overhead (when app submits a packet, the driver will send the
packet)
- Cons: hard to program (no more TCP stack), polling may waste cycles when packets aren't being
  received

Notes on IX/Arrakis

Problem IX is tackling:
- Why is it hard to achieve high throughput and low latency in TCP/IP stack?
    - Handling 1 packet a time is good for low latency
    - Batching is better for high load, but packets end up having higher latency
- What is kernel bypass networking?
    - User-level code has direct access to hardware
    - What about other users of NIC?
        - Like in Arrakis, SR-IOV lets us virtualize the NIC hardware
    - How do we protect important memory? (User shouldn’t be able to DMA some protected OS structure?
        - Use an IOMMU - IO memory management unit
            - How should I/O device read/write memory
            - Device should not be able to R/W arbitrary host memory
            - Same goes for guest VMs in case of virtualization
            - Job of IOMMU is to perform this memory mapping between device memory and host
                - Can be implemented w and w/o virtualization — w virtualization, implementation is similar to EPT we discussed last time
                - IOMMU also checks permissions
            - Pinning — must “pre-allocate” physical memory for each device to access and physically allocate space for it
                - Required because devices have no way to handle page faults
                - Recall: page fault happens when a virtual memory address is not in physical memory but is swapped out to disk
                    - What happens if a device reads/writes memory that is not present? Bad things
                    - Solution is to “pin” memory for each device