Range Matching TNA

layal · February 10, 2025, 10:11am

Hello,
I am trying to compile a program that includes range match key, and I encountered the following error:

"error: Currently in p4c, the table forward_l2_0 cannot perform a range match on key ingress::meta.current as the key does not fit in under 5 PHV nibbles.

Are there constraints on the key size for range matches? Also, is the range matching algorithm in tofino now open source?

p4prof · February 12, 2025, 4:11am

@layal ,

According to the message, you defined a table that uses range match kind on a key field that is longer than 20 bits (5 nibbles) and that is not supported by the compiler.

Unfortunately, I am not allowed to answer the rest of the questions in a public forum, since I am not aware of Intel ever publishing the range matching algorithm details. If you have access to ICRP Forum, please ask the question there so that I can provide more information.

It is my hope that sooner or later Intel will lift a lot of these restrictions, but currently this is not the case and your question clearly demonstrates the importance of doing that even (or especially) in light of open-sourcing the SDE.

layal · February 12, 2025, 8:12am

Thank you very much for your response.

fruffy · February 12, 2025, 1:54pm

Unfortunately, I am not allowed to answer the rest of the questions in a public forum, since I am not aware of Intel ever publishing the range matching algorithm details.

Considering much of the Tofino code is now open-source, we can probably use whatever exists in the code as our guideline

github.com/p4lang/p4c

backends/tofino/bf-p4c/mau/resource_estimate.cpp

c67d1fd01


      
              preferred_index = 0;
          
              for (auto &lo : layout_options) {
                  LOG3("entries " << lo.entries << " srams " << lo.srams << " tcams " << lo.tcams
                                  << " action data " << lo.layout.action_data_bytes_in_table << " immediate "
                                  << lo.layout.immediate_bits << " ternary indirect "
                                  << lo.layout.ternary_indirect_required());
              }
          }
          
          /** This pass calculates the number of lines an individual range match will take.  In order
           *  to perform a range match in Tofino, multiple lines of a TCAM may be necessary.  This is
           *  described in section 6.3.12 DirtCAM Example.  Essentially a larger range is broken into
           *  several smaller ranges that the hardware is capable of matching, which occur in factors
           *  of 16, as ranges are matched on a nibble by nibble basis, and 2^4 = 16.
           *
           *  The maximum number of rows that could be required by a range match is 2 * (nibbles involved
           *  in range match) - 1.  Let's look at a 12 bit range example 2 <= x <= 600
           *
           *  _____small_range_____|__nib_11_8__|__nib_7_4__|__nib_3_0__
           *      2 <= x <= 15     |      0     |     0     |  [2:15]

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Range Matching TNA

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