NetworkingTodo

This page should cover all networking related activity in KVM, currently most info is related to virtio-net.

TODO: add bugzilla entry links.

=== projects in progress. contributions are still very wellcome!

• vhost-net scalability tuning: threading for many VMs

     Plan: switch to workqueue shared by many VMs
     http://www.mail-archive.com/kvm@vger.kernel.org/msg69868.html

http://domino.research.ibm.com/library/cyberdig.nsf/1e4115aea78b6e7c85256b360066f0d4/479e3578ed05bfac85257b4200427735!OpenDocument

     Developer: Bandan Das
     Testing: netperf guest to guest

• multiqueue support in macvtap

      multiqueue is only supported for tun.
      Add support for macvtap.
      Developer: Jason Wang

• support more queues

    We limit TUN to 8 queues, but we really want
    1 queue per guest CPU. The limit comes from net
    core, need to teach it to allocate array of
    pointers and not array of queues.
    Jason has an draft patch to use flex array.
    Another thing is to move the flow caches out of tun_struct.
    Developer: Jason Wang

• enable multiqueue by default

      Multiqueue causes regression in some workloads, thus
      it is off by default. Detect and enable/disable
      automatically so we can make it on by default.
      This is because GSO tends to batch less when mq is enabled.
      https://patchwork.kernel.org/patch/2235191/
      Developer: Jason Wang

• rework on flow caches

      Current hlist implementation of flow caches has several limitations:
      1) at worst case, linear search will be bad
      2) not scale
      https://patchwork.kernel.org/patch/2025121/
      Developer: Jason Wang
      

• eliminate the extra copy in virtio-net driver

      We need do an extra copy of 128 bytes for every packets. 
      This could be eliminated for small packets by:
      1) use build_skb() and head frag
      2) bigger vnet header length ( >= NET_SKB_PAD + NET_IP_ALIGN )
      Or use a dedicated queue for small packet receiving ? (reordering)
      Developer: Jason Wang

• make pktgen works for virtio-net ( or partially orphan )

      virtio-net orphan the skb during tx,
      which will makes pktgen wait for ever to the refcnt.
      Jason's idea: introduce a flat to tell pktgen not for wait
      Discussion here: https://patchwork.kernel.org/patch/1800711/
      MST's idea: add a .ndo_tx_polling not only for pktgen
      Developer: Jason Wang

• Add HW_VLAN_TX support for tap

      Eliminate the extra data moving for tagged packets
      Developer: Jason Wang

• Announce self by guest driver

      Send gARP by guest driver. Guest part is finished.
      Qemu is ongoing.
      V7 patches is here:
      http://lists.nongnu.org/archive/html/qemu-devel/2013-03/msg01127.html
      Developer: Jason Wang

• guest programmable mac/vlan filtering with macvtap

       Developer: Amos Kong
       qemu: https://bugzilla.redhat.com/show_bug.cgi?id=848203
       libvirt: https://bugzilla.redhat.com/show_bug.cgi?id=848199
       https://git.kernel.org/cgit/virt/kvm/mst/qemu.git/patch/?id=1c0fa6b709d02fe4f98d4ce7b55a6cc3c925791c
       Status: GuestProgrammableMacVlanFiltering

• bridge without promisc mode in NIC

 given hardware support, teach bridge
 to program mac/vlan filtering in NIC
 Helps performance and security on noisy LANs
 http://comments.gmane.org/gmane.linux.network/266546
 Developer: Vlad Yasevich

• reduce networking latency:

 allow handling short packets from softirq or VCPU context
 Plan:
   We are going through the scheduler 3 times
   (could be up to 5 if softirqd is involved)
   Consider RX: host irq -> io thread -> VCPU thread ->
   guest irq -> guest thread.
   This adds a lot of latency.
   We can cut it by some 1.5x if we do a bit of work
   either in the VCPU or softirq context.
 Testing: netperf TCP RR - should be improved drastically
          netperf TCP STREAM guest to host - no regression
 Developer: MST

• Flexible buffers: put virtio header inline with packet data

 https://patchwork.kernel.org/patch/1540471/
 Developer: MST

• device failover to allow migration with assigned devices

 https://fedoraproject.org/wiki/Features/Virt_Device_Failover
 Developer: Gal Hammer, Cole Robinson, Laine Stump, MST

• Reuse vringh code for better maintainability

 Developer: Rusty Russell

• Improve stats, make them more helpful for per analysis

 Developer: Sriram Narasimhan

• Bug: e1000 & rtl8139: Change macaddr in guest, but not update to qemu (info network)

 Developer: Amos Kong
 https://bugzilla.redhat.com/show_bug.cgi?id=922589

• Enable GRO for packets coming to bridge from a tap interface

 Developer: Dmitry Fleytman

• Better support for windows LRO

 Extend virtio-header with statistics for GRO packets:
 number of packets coalesced and number of duplicate ACKs coalesced
 Developer: Dmitry Fleytman

• IPoIB infiniband bridging

 Plan: implement macvtap for ipoib and virtio-ipoib
 Developer: MST

• netdev polling for virtio.

 There are two kinds of netdev polling:
 - netpoll - used for debugging
 - proposed low latency net polling
 See http://lkml.indiana.edu/hypermail/linux/kernel/1303.0/00553.html
 Developer: Jason Wang

• sharing config interrupts

 Support mode devices by sharing a single msi vector
 between multiple virtio devices.
 (Applies to virtio-blk too).
 Developer: Amos Kong

• use kvm eventfd support for injecting level interrupts,

 enable vhost by default for level interrupts
 Developer: Amos Kong

projects that are not started yet - no owner

• receive side zero copy

 The ideal is a NIC with accelerated RFS support,
 So we can feed the virtio rx buffers into the correct NIC queue.
 Depends on non promisc NIC support in bridge.
 Search for "Xin Xiaohui: Provide a zero-copy method on KVM virtio-net"
 for a very old prototype

• RDMA bridging

• DMA emgine (IOAT) use in tun

 Old patch here: [PATCH RFC] tun: dma engine support
 It does not speed things up. Need to see why and
 what can be done.

• virtio API extension: improve small packet/large buffer performance:

 support "reposting" buffers for mergeable buffers,
 support pool for indirect buffers

• more GSO type support:

      Kernel not support more type of GSO: FCOE, GRE, UDP_TUNNEL

• ring aliasing:

 using vhost-net as a networking backend with virtio-net in QEMU
 being what's guest facing.
 This gives you the best of both worlds: QEMU acts as a first
 line of defense against a malicious guest while still getting the
 performance advantages of vhost-net (zero-copy).
 In fact a bit of complexity in vhost was put there in the vague hope to
 support something like this: virtio rings are not translated through
 regular memory tables, instead, vhost gets a pointer to ring address.
 This allows qemu acting as a man in the middle,
 verifying the descriptors but not touching the packet data.

• non-virtio device support with vhost

 Use vhost interface for guests that don't use virtio-net

=== vague ideas: path to implementation not clear

• ring redesign:

     find a way to test raw ring performance 
     fix cacheline bounces 
     reduce interrupts

• irq/numa affinity:

    networking goes much faster with irq pinning:
    both with and without numa.
    what can be done to make the non-pinned setup go faster?

• reduce conflict with VCPU thread

   if VCPU and networking run on same CPU,
   they conflict resulting in bad performance.
   Fix that, push vhost thread out to another CPU
   more aggressively.

• rx mac filtering in tun

       the need for this is still not understood as we have filtering in bridge
       we have a small table of addresses, need to make it larger
       if we only need filtering for unicast (multicast is handled by IMP filtering)

• vlan filtering in tun

       the need for this is still not understood as we have filtering in bridge

• vlan filtering in bridge

       kernel part is done (Vlad Yasevich)
       teach qemu to notify libvirt to enable the filter (still to do) (existed NIC_RX_FILTER_CHANGED event contains vlan-tables)

• tx coalescing

       Delay several packets before kick the device.

• interrupt coalescing

       Reduce the number of interrupt

• bridging on top of macvlan

 add code to forward LRO status from macvlan (not macvtap)
 back to the lowerdev, so that setting up forwarding
 from macvlan disables LRO on the lowerdev

• virtio: preserve packets exactly with LRO

 LRO is not normally compatible with forwarding.
 virtio we are getting packets from a linux host,
 so we could thinkably preserve packets exactly
 even with LRO. I am guessing other hardware could be
 doing this as well.

• vxlan

 What could we do here?

• bridging without promisc mode with OVS

testing projects

Keeping networking stable is highest priority.

• Write some unit tests for vhost-net/vhost-scsi
• Run weekly test on upstream HEAD covering test matrix with autotest
• Measure the effect of each of the above-mentioned optimizations

 - Use autotest network performance regression testing (that runs netperf)
 - Also test any wild idea that works. Some may be useful.

• Migrate some of the performance regression autotest functionality into Netperf

 - Get the CPU-utilization of the Host and the other-party, and add them to the report. This is also true for other Host measures, such as vmexits, interrupts, ...
 - Run Netperf in demo-mode, and measure only the time when all the sessions are active (could be many seconds after the beginning of the tests)
 - Packaging of Netperf in Fedora / RHEL (exists in Fedora). Licensing could be an issue.
 - Make the scripts more visible

non-virtio-net devices

• e1000: stabilize

test matrix

DOA test matrix (all combinations should work):

       vhost: test both on and off, obviously
       test: hotplug/unplug, vlan/mac filtering, netperf,
            file copy both ways: scp, NFS, NTFS
       guests: linux: release and debug kernels, windows
       conditions: plain run, run while under migration,
               vhost on/off migration
       networking setup: simple, qos with cgroups
       host configuration: host-guest, external-guest