[PATCH bpf-next v2] bpf, docs: Clarify PC use in instruction-set.rst

[Dave Thaler <dthaler1968@googlemail.com>]

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.

v1->v2: reword per feedback from Alexei

Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
---
 Documentation/bpf/standardization/instruction-set.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index b44bdacd0..766f57636 100644
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/Documentation/bpf/standardization/instruction-set.rst
@@ -469,6 +469,12 @@ JSLT      0xc    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 and fails verification 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
 ``EXIT``.
 
-- 
2.40.1