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diff --git a/docs/reference/multiarch/nodefns.rst b/docs/reference/multiarch/nodefns.rst deleted file mode 100644 index ad68385d6c5..00000000000 --- a/docs/reference/multiarch/nodefns.rst +++ /dev/null @@ -1,160 +0,0 @@ -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. |