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diff --git a/src/vnet/gso/gso.rst b/src/vnet/gso/gso.rst new file mode 100644 index 00000000000..78788f82216 --- /dev/null +++ b/src/vnet/gso/gso.rst @@ -0,0 +1,154 @@ +.. _gso_doc: + +Generic Segmentation Offload +============================ + +Overview +________ + +Modern physical NICs provide offload capabilities to software based network +stacks to transfer some type of the packet processing from CPU to physical +NICs. TCP Segmentation Offload (TSO) is one among many which is provided by +modern physical NICs. Software based network stack can offload big (up to 64KB) +TCP packets to NIC and NIC will segment them into Maximum Segment Size packets. +Hence network stack save CPU cycles by processing few big packets instead of +processing many small packets. + +GSO is software based analogous to TSO which is used by virtual interfaces +i.e. tap, virtio, af_packet, vhost-user etc. Typically, virtual interfaces +provide capability to offload big packets (64KB size). But in reality, they +just pass the packet as it is to the other end without segmenting it. Hence, it +is necessary to validate the support of GSO offloading in whole setup otherwise +packet will be dropped when it will be processed by virtual entity which does +not support GSO. + +The GSO Infrastructure +_______________________ + +Software based network stacks implements GSO packet segmentation in software +where egress interface (virtual or physical) does not support GSO or TSO +offload. VPP implements GSO stack to provide support for software based packet +chunking of GSO packets when egress interface does not support GSO or TSO +offload. + +It is implemented as a feature node on interface-output feature arc. It +implements support for basic GSO, GSO with VXLAN tunnel and GSO with IPIP +tunnel. GSO with Geneve and GSO with NVGRE are not supported today. But one can +enable GSO feature node on tunnel interfaces i.e. IPSEC etc to segment GSO +packets before they will be tunneled. + +Virtual interfaces does not support GSO with tunnels. So, special care is +needed when user configures tunnel(s) along with GSO in the setup. In such case, +either enable GSO feature node on tunnel interface (mean chunk the GSO packets +before they will be encapsulated in tunnel) or disable the GSO offload on the +egress interface (only work for VXLAN tunnel and IPIP tunnel), if it is enabled, +should work fine. + +Similarly, many physical interfaces does not support GSO with tunnels too. User +can do the same configuration as it is mentioned previously for virtual +interfaces. + +Data structures +^^^^^^^^^^^^^^^ + +VPP ``vlib_buffer_t`` uses ``VNET_BUFFER_F_GSO`` flags to mark the buffer carrying GSO +packet and also contain metadata fields with respect to GSO: + +.. code:: c + + i16 l2_hdr_offset; + i16 l3_hdr_offset; + i16 l4_hdr_offset; + + u16 gso_size; + u16 gso_l4_hdr_sz; + i16 outer_l3_hdr_offset; + i16 outer_l4_hdr_offset; + +Packet header offsets are computed from the reference of ``vlib_buffer_t`` data +pointer. + +``l2_hdr_offset``, ``l3_hdr_offset`` and ``l4_hdr_offset`` are set on input of checksum +offload or GSO enabled interfaces or features i.e. host stack. Appropriate +offload flags are also set to ``vnet_buffer_oflags_t`` to reflect the actual packet +offloads which will be used later at egress interface tx node or +interface-output node or GSO node to process the packet appropriately. These +fields are present in 1st cache line and does not incur extra cycles as most of +the VPP features fetch the ``vlib_buffer_t`` 1st cache line to access ``current_data`` +or ``current_length`` fields of the packet. + +Please note that ``gso_size``, ``gso_l4_hdr_sz``, ``outer_l3_hdr_offset`` and +``outer_l4_hdr_offset`` are in second cache line of ``vlib_buffer_t``. Accessing them in +data plane will incur some extra cycles but cost of these cycles will be +amortized over (up to 64KB) packet. + +The ``gso_size`` and ``gso_l4_hdr_sz`` are set on input of GSO enabled interfaces (tap, +virtio, af_packet etc) or features (vpp host stack), when we receive a GSO +packet (a chain of buffers with the first one having ``VNET_BUFFER_F_GSO`` bit set), +and needs to persist all the way to the interface-output, in case the egress +interface is not GSO-enabled - then we need to perform the segmentation, and use +these values to chunk the payload appropriately. + +``outer_l3_hdr_offset`` and ``outer_l4_hdr_offset`` are used in case of tunneled packet +(i.e. VXLAN or IPIP). ``outer_l3_hdr_offset`` will point to outer l3 header of the +tunnel headers and ``outer_l4_hdr_offset`` will point to outer l4 header of the +tunnel headers, if any. + +Following are the helper functions used to set and clear the offload flags from +``vlib_buffer_t`` metadata: + +.. code:: c + + static_always_inline void + vnet_buffer_offload_flags_set (vlib_buffer_t *b, vnet_buffer_oflags_t oflags) + { + if (b->flags & VNET_BUFFER_F_OFFLOAD) + { + /* add a flag to existing offload */ + vnet_buffer (b)->oflags |= oflags; + } + else + { + /* no offload yet: reset offload flags to new value */ + vnet_buffer (b)->oflags = oflags; + b->flags |= VNET_BUFFER_F_OFFLOAD; + } + } + + static_always_inline void + vnet_buffer_offload_flags_clear (vlib_buffer_t *b, vnet_buffer_oflags_t oflags) + { + vnet_buffer (b)->oflags &= ~oflags; + if (0 == vnet_buffer (b)->oflags) + b->flags &= ~VNET_BUFFER_F_OFFLOAD; + } + + +ENABLE GSO FEATURE NODE +----------------------- + +GSO feature node is not enabled by default when egress interface does not +support GSO. User has to enable it explicitly using api or cli. + +GSO API +^^^^^^^ + +This API message is used to enable GSO feature node on an interface. + +.. code:: c + + autoreply define feature_gso_enable_disable + { + u32 client_index; + u32 context; + vl_api_interface_index_t sw_if_index; + bool enable_disable; + option vat_help = "<intfc> | sw_if_index <nn> [enable | disable]"; + }; + +GSO CLI +^^^^^^^ + +:: + + set interface feature gso <intfc> [enable | disable] |