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authorChristoph Hellwig <hch@lst.de>2022-01-31 21:36:37 +0300
committerAlexei Starovoitov <ast@kernel.org>2022-02-03 20:29:37 +0300
commit5ca15b8a939ff59f3e9f957ad4af5658c016cf7e (patch)
tree873e03d024bd953dda7b3790279d1785cd4d54ce
parent15175336270a76695412aedf68f3eab746d84b4b (diff)
downloadlinux-5ca15b8a939ff59f3e9f957ad4af5658c016cf7e.tar.xz
bpf, docs: Better document the extended instruction format
In addition to the normal 64-bit instruction encoding, eBPF also has a single instruction that uses a second 64-bit bits for a second immediate value. Instead of only documenting this format deep down in the document mention it in the instruction encoding section. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20220131183638.3934982-5-hch@lst.de
-rw-r--r--Documentation/bpf/instruction-set.rst26
1 files changed, 19 insertions, 7 deletions
diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 94682e23cd2f..a8e9fced39ad 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -22,7 +22,13 @@ necessary across calls.
Instruction encoding
====================
-eBPF uses 64-bit instructions with the following encoding:
+eBPF has two instruction encodings:
+
+ * the basic instruction encoding, which uses 64 bits to encode an instruction
+ * the wide instruction encoding, which appends a second 64-bit immediate value
+ (imm64) after the basic instruction for a total of 128 bits.
+
+The basic instruction encoding looks as follows:
============= ======= =============== ==================== ============
32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
@@ -212,7 +218,7 @@ The mode modifier is one of:
============= ===== ====================================
mode modifier value description
============= ===== ====================================
- BPF_IMM 0x00 used for 64-bit mov
+ BPF_IMM 0x00 64-bit immediate instructions
BPF_ABS 0x20 legacy BPF packet access (absolute)
BPF_IND 0x40 legacy BPF packet access (indirect)
BPF_MEM 0x60 regular load and store operations
@@ -287,12 +293,18 @@ 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.
-16-byte instructions
---------------------
+64-bit immediate instructions
+-----------------------------
+
+Instructions with the ``BPF_IMM`` mode modifier use the wide instruction
+encoding for an extra imm64 value.
+
+There is currently only one such instruction.
+
+``BPF_LD | BPF_DW | BPF_IMM`` means::
+
+ dst_reg = imm64
-eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists
-of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single
-instruction that loads 64-bit immediate value into a dst_reg.
Legacy BPF Packet access instructions
-------------------------------------