Commit de948576 authored by Brendan Jackman's avatar Brendan Jackman Committed by Alexei Starovoitov
Browse files

bpf: Document new atomic instructions 



Document new atomic instructions.

Signed-off-by: default avatarBrendan Jackman <jackmanb@google.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210114181751.768687-12-jackmanb@google.com
parent 98d666d0

Documentation/networking/filter.rst

+31 −0
Original line number Diff line number Diff line
@@ -1053,8 +1053,39 @@ encoding. 
   .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg

   .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg



The basic atomic operations supported are:



    BPF_ADD

    BPF_AND

    BPF_OR

    BPF_XOR



Each having equivalent semantics with the ``BPF_ADD`` example, that is: the

memory location addresed by ``dst_reg + off`` is atomically modified, with

``src_reg`` as the other operand. If the ``BPF_FETCH`` flag is set in the

immediate, then these operations also overwrite ``src_reg`` with the

value that was in memory before it was modified.



The more special operations are:



    BPF_XCHG



This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +

off``.



    BPF_CMPXCHG



This atomically compares the value addressed by ``dst_reg + off`` with

``R0``. If they match it is replaced with ``src_reg``, The value that was there

before is loaded back to ``R0``.



Note that 1 and 2 byte atomic operations are not supported.



Except ``BPF_ADD`` _without_ ``BPF_FETCH`` (for legacy reasons), all 4 byte

atomic operations require alu32 mode. Clang enables this mode by default in

architecture v3 (``-mcpu=v3``). For older versions it can be enabled with

``-Xclang -target-feature -Xclang +alu32``.



You may encounter BPF_XADD - this is a legacy name for BPF_ATOMIC, referring to

the exclusive-add operation encoded when the immediate field is zero.