[PATCH bpf-next v3] bpf, docs: Clarify PC use in instruction-set.rst

From: Dave Thaler <dthaler1968@googlemail.com>
To: bpf@vger.kernel.org
Cc: bpf@ietf.org, Dave Thaler <dthaler1968@gmail.com>,
	Dave Thaler <dthaler1968@googlemail.com>
Subject: [PATCH bpf-next v3] bpf, docs: Clarify PC use in instruction-set.rst
Date: Fri, 26 Apr 2024 16:11:26 -0700	[thread overview]
Message-ID: <20240426231126.5130-1-dthaler1968@gmail.com> (raw)

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

v2->v3: reword per feedback from David Vernet

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..997560aba 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 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
 ``EXIT``.
 
-- 
2.40.1

