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bpf, docs: Clarify PC use in instruction-set.rst

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This patch elaborates on the use of PC by expanding the PC acronym,
explaining the units, and the relative position to which the offset
applies.

Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240426231126.5130-1-dthaler1968@gmail.com
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Dave Thaler 2024-04-26 16:11:26 -07:00 committed by Daniel Borkmann
parent 6e25bcf06a
commit 07801a24e2
1 changed files with 6 additions and 0 deletions
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Documentation/bpf/standardization/instruction-set.rst
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@ -469,6 +469,12 @@ JSLT 0xc any PC += offset if dst < src signed
JSLE 0xd any PC += offset if dst <= src signed JSLE 0xd any PC += offset if dst <= src signed
======== ===== ======= ================================= =================================================== ======== ===== ======= ================================= ===================================================
where 'PC' denotes the program counter, and the offset to increment by
is in units of 64-bit instructions relative to the instruction following
the jump instruction. Thus 'PC += 1' skips execution of the next
instruction if it's a basic instruction or results in undefined behavior
if the next instruction is a 128-bit wide instruction.
The BPF program needs to store the return value into register R0 before doing an The BPF program needs to store the return value into register R0 before doing an
``EXIT``. ``EXIT``.