Documentation/bpf: Add documentation for BPF_PROG_RUN

This adds documentation for the BPF_PROG_RUN command; a short overview of
the command itself, and a more verbose description of the "live packet"
mode for XDP introduced in the previous commit.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220309105346.100053-3-toke@redhat.com

Documentation/bpf/bpf_prog_run.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Running BPF programs from userspace
+===================================
+
+This document describes the ``BPF_PROG_RUN`` facility for running BPF programs
+from userspace.
+
+.. contents::
+    :local:
+    :depth: 2
+
+
+Overview
+--------
+
+The ``BPF_PROG_RUN`` command can be used through the ``bpf()`` syscall to
+execute a BPF program in the kernel and return the results to userspace. This
+can be used to unit test BPF programs against user-supplied context objects, and
+as way to explicitly execute programs in the kernel for their side effects. The
+command was previously named ``BPF_PROG_TEST_RUN``, and both constants continue
+to be defined in the UAPI header, aliased to the same value.
+
+The ``BPF_PROG_RUN`` command can be used to execute BPF programs of the
+following types:
+
+- ``BPF_PROG_TYPE_SOCKET_FILTER``
+- ``BPF_PROG_TYPE_SCHED_CLS``
+- ``BPF_PROG_TYPE_SCHED_ACT``
+- ``BPF_PROG_TYPE_XDP``
+- ``BPF_PROG_TYPE_SK_LOOKUP``
+- ``BPF_PROG_TYPE_CGROUP_SKB``
+- ``BPF_PROG_TYPE_LWT_IN``
+- ``BPF_PROG_TYPE_LWT_OUT``
+- ``BPF_PROG_TYPE_LWT_XMIT``
+- ``BPF_PROG_TYPE_LWT_SEG6LOCAL``
+- ``BPF_PROG_TYPE_FLOW_DISSECTOR``
+- ``BPF_PROG_TYPE_STRUCT_OPS``
+- ``BPF_PROG_TYPE_RAW_TRACEPOINT``
+- ``BPF_PROG_TYPE_SYSCALL``
+
+When using the ``BPF_PROG_RUN`` command, userspace supplies an input context
+object and (for program types operating on network packets) a buffer containing
+the packet data that the BPF program will operate on. The kernel will then
+execute the program and return the results to userspace. Note that programs will
+not have any side effects while being run in this mode; in particular, packets
+will not actually be redirected or dropped, the program return code will just be
+returned to userspace. A separate mode for live execution of XDP programs is
+provided, documented separately below.
+
+Running XDP programs in "live frame mode"
+-----------------------------------------
+
+The ``BPF_PROG_RUN`` command has a separate mode for running live XDP programs,
+which can be used to execute XDP programs in a way where packets will actually
+be processed by the kernel after the execution of the XDP program as if they
+arrived on a physical interface. This mode is activated by setting the
+``BPF_F_TEST_XDP_LIVE_FRAMES`` flag when supplying an XDP program to
+``BPF_PROG_RUN``.
+
+The live packet mode is optimised for high performance execution of the supplied
+XDP program many times (suitable for, e.g., running as a traffic generator),
+which means the semantics are not quite as straight-forward as the regular test
+run mode. Specifically:
+
+- When executing an XDP program in live frame mode, the result of the execution
+  will not be returned to userspace; instead, the kernel will perform the
+  operation indicated by the program's return code (drop the packet, redirect
+  it, etc). For this reason, setting the ``data_out`` or ``ctx_out`` attributes
+  in the syscall parameters when running in this mode will be rejected. In
+  addition, not all failures will be reported back to userspace directly;
+  specifically, only fatal errors in setup or during execution (like memory
+  allocation errors) will halt execution and return an error. If an error occurs
+  in packet processing, like a failure to redirect to a given interface,
+  execution will continue with the next repetition; these errors can be detected
+  via the same trace points as for regular XDP programs.
+
+- Userspace can supply an ifindex as part of the context object, just like in
+  the regular (non-live) mode. The XDP program will be executed as though the
+  packet arrived on this interface; i.e., the ``ingress_ifindex`` of the context
+  object will point to that interface. Furthermore, if the XDP program returns
+  ``XDP_PASS``, the packet will be injected into the kernel networking stack as
+  though it arrived on that ifindex, and if it returns ``XDP_TX``, the packet
+  will be transmitted *out* of that same interface. Do note, though, that
+  because the program execution is not happening in driver context, an
+  ``XDP_TX`` is actually turned into the same action as an ``XDP_REDIRECT`` to
+  that same interface (i.e., it will only work if the driver has support for the
+  ``ndo_xdp_xmit`` driver op).
+
+- When running the program with multiple repetitions, the execution will happen
+  in batches. The batch size defaults to 64 packets (which is same as the
+  maximum NAPI receive batch size), but can be specified by userspace through
+  the ``batch_size`` parameter, up to a maximum of 256 packets. For each batch,
+  the kernel executes the XDP program repeatedly, each invocation getting a
+  separate copy of the packet data. For each repetition, if the program drops
+  the packet, the data page is immediately recycled (see below). Otherwise, the
+  packet is buffered until the end of the batch, at which point all packets
+  buffered this way during the batch are transmitted at once.
+
+- When setting up the test run, the kernel will initialise a pool of memory
+  pages of the same size as the batch size. Each memory page will be initialised
+  with the initial packet data supplied by userspace at ``BPF_PROG_RUN``
+  invocation. When possible, the pages will be recycled on future program
+  invocations, to improve performance. Pages will generally be recycled a full
+  batch at a time, except when a packet is dropped (by return code or because
+  of, say, a redirection error), in which case that page will be recycled
+  immediately. If a packet ends up being passed to the regular networking stack
+  (because the XDP program returns ``XDP_PASS``, or because it ends up being
+  redirected to an interface that injects it into the stack), the page will be
+  released and a new one will be allocated when the pool is empty.
+
+  When recycling, the page content is not rewritten; only the packet boundary
+  pointers (``data``, ``data_end`` and ``data_meta``) in the context object will
+  be reset to the original values. This means that if a program rewrites the
+  packet contents, it has to be prepared to see either the original content or
+  the modified version on subsequent invocations.

Documentation/bpf/index.rst

@@ -21,6 +21,7 @@ that goes into great technical depth about the BPF Architecture.
    helpers
    programs
    maps
+   bpf_prog_run
    classic_vs_extended.rst
    bpf_licensing
    test_debug