LPM Match Fields with Bitwidth Not Divisible by 8. #146
Description
Hi, seen this issue with match fields with bitwidth % 8 !=0.
When setting match value using runtime shell, it seems like runtime shell expects values to be calculated aligned to full bytes, for example, for a field with bitwidth = 2 match on "0x1/2" need to be translated to "0x80/2". If not, value is seen as 0, causing 2 entries with different match keys to seem like they have the same key:
Keys 1 and 3 regarded both as zeros:
P4Runtime sh >>> te.match["MY_FIELD"] = "0x1/2"
LPM value was transformed to conform to the P4Runtime spec (trailing bits must be unset)
field_id: 1
lpm {
value: "\000" # Zeros
prefix_len: 2
}
P4Runtime sh >>> te.match["MY_FIELD"] = "0x3/2"
LPM value was transformed to conform to the P4Runtime spec (trailing bits must be unset)
field_id: 1
lpm {
value: "\000" # Zeros
prefix_len: 2
}
One can also avoid it by using a different prefix length, let's say instead of `"0x1/2", use "0x1/8" to be byte aligned.
In both cases, we get an error that size of value or prefix length does not fit the field info.
Looking into code, it seems that byte_mask = 0xff & ((0xff << (8 - r))) does create the mask from start of byte and without regarding the bitwidth, meaning for a field with bitwidth = 2 and prefix_len = 2 the mask will be 0b11000000.
It is debatable whether it makes more sense to have the prefix length as the real length and not completed to a byte.
So question is - is this expected? Should I try and implment a fix?
Some logs:
> /usr/local/lib/python3.12/dist-packages/p4runtime_sh/shell.py(428)_sanitize_and_convert_mf_lpm()
-> if length == 0:
(Pdb) prefix
b'\x01'
(Pdb) length
2
(Pdb) field_info
id: 1
name: "MY_FIELD"
bitwidth: 2
match_type: LPM
(Pdb) n
> /usr/local/lib/python3.12/dist-packages/p4runtime_sh/shell.py(445)_sanitize_and_convert_mf_lpm()
-> if barray[first_byte_masked] & byte_mask != barray[first_byte_masked]:
(Pdb) byte_mask
192
(Pdb) bin(192)
'0b11000000'