DOC-ONLY: document latest multi-arch support scheme

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I613415c109f648248ee04dd76d5e652dbf21bc7f

2 files changed, 171 insertions, 0 deletions

diff --git a/docs/reference/multiarch/index.rst b/docs/reference/multiarch/index.rst
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+.. _multiarch:
+
+Multi-architecture support
+==========================
+
+This reference guide describes how to use the vpp muli-architecture support scheme
+
+.. toctree::
+   :maxdepth: 1
+
+   nodefns
diff --git a/docs/reference/multiarch/nodefns.rst b/docs/reference/multiarch/nodefns.rst
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+Multi-Architecture Graph Node Cookbook
+======================================
+
+In the context of graph node dispatch functions, it's easy enough to
+use the vpp multi-architecture support setup. The point of the scheme
+is simple: for performance-critical nodes, generate multiple CPU
+hardware-dependent versions of the node dispatch functions, and pick
+the best one at runtime.
+
+The vpp scheme is simple enough to use, but details matter.
+
+100,000 foot view
+-----------------
+
+We compile entire graph node dispatch function implementation files
+multiple times. These compilations give rise to multiple versions of
+the graph node dispatch functions. Per-node constructor-functions
+interrogate CPU hardware, select the node dispatch function variant to
+use, and set the vlib_node_registration_t ".function" member to the
+address of the selected variant.
+
+Details
+-------
+
+Declare the node dispatch function as shown, using the VLIB\_NODE\_FN macro. The
+name of the node function **MUST** match the name of the graph node. 
+
+:: 
+
+    VLIB_NODE_FN (ip4_sdp_node) (vlib_main_t * vm, vlib_node_runtime_t * node,
+                                 vlib_frame_t * frame)
+    {
+      if (PREDICT_FALSE (node->flags & VLIB_NODE_FLAG_TRACE))
+        return ip46_sdp_inline (vm, node, frame, 1 /* is_ip4 */ ,
+    			    1 /* is_trace */ );
+      else
+        return ip46_sdp_inline (vm, node, frame, 1 /* is_ip4 */ ,
+    			    0 /* is_trace */ );
+    }   
+
+We need to generate *precisely one copy* of the
+vlib_node_registration_t, error strings, and packet trace decode function.
+
+Simply bracket these items with "#ifndef CLIB_MARCH_VARIANT...#endif":
+
+::
+
+    #ifndef CLIB_MARCH_VARIANT
+    static u8 *
+    format_sdp_trace (u8 * s, va_list * args)
+    {
+       <snip>
+    }
+    #endif
+
+    ...
+
+    #ifndef CLIB_MARCH_VARIANT
+    static char *sdp_error_strings[] = {
+    #define _(sym,string) string,
+      foreach_sdp_error
+    #undef _
+    };
+    #endif
+
+    ...
+
+    #ifndef CLIB_MARCH_VARIANT
+    VLIB_REGISTER_NODE (ip4_sdp_node) =
+    {
+      // DO NOT set the .function structure member.
+      // The multiarch selection __attribute__((constructor)) function
+      // takes care of it at runtime
+      .name = "ip4-sdp",
+      .vector_size = sizeof (u32),
+      .format_trace = format_sdp_trace,
+      .type = VLIB_NODE_TYPE_INTERNAL,
+
+      .n_errors = ARRAY_LEN(sdp_error_strings),
+      .error_strings = sdp_error_strings,
+
+      .n_next_nodes = SDP_N_NEXT,
+
+      /* edit / add dispositions here */
+      .next_nodes =
+      {
+        [SDP_NEXT_DROP] = "ip4-drop",
+      },
+    };
+    #endif
+
+To belabor the point: *do not* set the ".function" member! That's the job of the multi-arch
+selection \_\_attribute\_\_((constructor)) function
+
+Always inline node dispatch functions
+-------------------------------------
+
+It's typical for a graph dispatch function to contain one or more
+calls to an inline function. See above. If your node dispatch function
+is structured that way, make *ABSOLUTELY CERTAIN* to use the
+"always_inline" macro:
+
+::
+
+    always_inline uword
+    ip46_sdp_inline (vlib_main_t * vm, vlib_node_runtime_t * node, 
+                 vlib_frame_t * frame,
+    		 int is_ip4, int is_trace)
+    { ... }
+
+Otherwise, the compiler is highly likely NOT to build multiple
+versions of the guts of your dispatch function. 
+
+It's fairly easy to spot this mistake in "perf top." If you see, for
+example, a bunch of functions with names of the form
+"xxx_node_fn_avx2" in the profile, *BUT* your brand-new node function
+shows up with a name of the form "xxx_inline.isra.1", it's quite likely
+that the inline was declared "static inline" instead of "always_inline".
+
+Add the required Makefile.am content
+------------------------------------
+
+If the component in question already sets a "multiversioning_sources"
+variable, simply add the indicated .c file to the list. If not, add
+the required boilerplate:
+
+::
+
+    if CPU_X86_64
+    sdp_multiversioning_sources =			\
+    	sdp/node.c				\
+    	sdp/sdp_slookup.c
+
+    if CC_SUPPORTS_AVX2
+    ###############################################################
+    # AVX2
+    ###############################################################
+    libsdp_plugin_avx2_la_SOURCES = $(sdp_multiversioning_sources)
+    libsdp_plugin_avx2_la_CFLAGS =					\
+    	$(AM_CFLAGS)  @CPU_AVX2_FLAGS@				\
+    	-DCLIB_MARCH_VARIANT=avx2
+    noinst_LTLIBRARIES += libsdp_plugin_avx2.la
+    sdp_plugin_la_LIBADD += libsdp_plugin_avx2.la
+    endif
+
+    if CC_SUPPORTS_AVX512
+    ###############################################################
+    # AVX512
+    ###############################################################
+    libsdp_plugin_avx512_la_SOURCES = $(sdp_multiversioning_sources)
+    libsdp_plugin_avx512_la_CFLAGS =				\
+    	$(AM_CFLAGS) @CPU_AVX512_FLAGS@				\
+    	-DCLIB_MARCH_VARIANT=avx512
+    noinst_LTLIBRARIES += libsdp_plugin_avx512.la
+    sdp_plugin_la_LIBADD += libsdp_plugin_avx512.la
+    endif
+    endif
+
+A certain amount of cut-paste-modify is currently required. Hopefully
+we'll manage to improve the scheme in the future.