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Diffstat (limited to 'src/plugins/mdata/mdata_doc.rst')
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diff --git a/src/plugins/mdata/mdata_doc.rst b/src/plugins/mdata/mdata_doc.rst new file mode 100644 index 00000000000..95746bd3d0e --- /dev/null +++ b/src/plugins/mdata/mdata_doc.rst @@ -0,0 +1,26 @@ +Buffer metadata change tracker +============================== + +Introduction +------------ + +The mdata plugin uses the vlib main loop “before” performance counter +hook to snapshoot buffer metadata before calling the node dispatch +function. Similarly, the plugin uses the main loop “after” hook to +compare a vectors’ worth of buffer metadata after the fact. + +The comparison function is a simple octet-by-octet A != B check. We +accumulate changed octets per-node across the entire run, using a single +spinlock-protected accumulator. + +The “show buffer metadata” command produces a report of all fields whose +values are changed by nodes visited during a given run. + +Since many fields in the vnet_buffer_opaque_t are union members, it may +appear that a certain node changes numerous fields. The entire point of +the exercise is to warn developers that if a packet visits node N, data +placed into opaque union field F *will* be affected. + +One should never assume much about buffer metadata immutability across +arbitrary subgraphs. This tool generates accurate reports, to the extent +that one exercises the required subgraph trajectories. |