gas/doc/c-bpf.texi

@c Copyright (C) 2019-2021 Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.

@ifset GENERIC
@page
@node BPF-Dependent
@chapter BPF Dependent Features
@end ifset

@ifclear GENERIC
@node Machine Dependencies
@chapter BPF Dependent Features
@end ifclear

@cindex BPF support
@menu
* BPF Options::                 Options
* BPF Syntax::		        Syntax
* BPF Directives::		Machine Directives
* BPF Opcodes::			Opcodes
@end menu

@node BPF Options
@section Options
@cindex BPF options (none)
@cindex options for BPF (none)

@c man begin OPTIONS
@table @gcctabopt

@cindex @option{-EB} command-line option, BPF
@item -EB
This option specifies that the assembler should emit big-endian eBPF.

@cindex @option{-EL} command-line option, BPF
@item -EL
This option specifies that the assembler should emit little-endian
eBPF.
@end table

Note that if no endianness option is specified in the command line,
the host endianness is used.
@c man end

@node BPF Syntax
@section Syntax
@menu
* BPF-Chars::                Special Characters
* BPF-Regs::                 Register Names
* BPF-Pseudo-Maps::	     Pseudo map fds
@end menu

@node BPF-Chars
@subsection Special Characters

@cindex line comment character, BPF
@cindex BPF line comment character
The presence of a @samp{;} on a line indicates the start of a comment
that extends to the end of the current line.  If a @samp{#} appears as
the first character of a line, the whole line is treated as a comment.

@cindex statement separator, BPF
Statements and assembly directives are separated by newlines.

@node BPF-Regs
@subsection Register Names

@cindex BPF register names
@cindex register names, BPF
The eBPF processor provides ten general-purpose 64-bit registers,
which are read-write, and a read-only frame pointer register:

@table @samp
@item %r0 .. %r9
General-purpose registers.
@item %r10
Frame pointer register.
@end table

Some registers have additional names, to reflect their role in the
eBPF ABI:

@table @samp
@item %a
This is @samp{%r0}.
@item %ctx
This is @samp{%r6}.
@item %fp
This is @samp{%r10}.
@end table

@node BPF-Pseudo-Maps
@subsection Pseudo Maps

@cindex pseudo map fd, BPF
The @samp{LDDW} instruction can take a literal pseudo map file
descriptor as its second argument.  This uses the syntax
@samp{%map_fd(N)} where @samp{N} is a signed number.

For example, to load the address of the pseudo map with file
descriptor @samp{2} in register @samp{r1} we would do:

@smallexample
        lddw %r1, %map_fd(2)
@end smallexample

@node BPF Directives
@section Machine Directives

@cindex machine directives, BPF

The BPF version of @code{@value{AS}} supports the following additional
machine directives:

@table @code
@cindex @code{half} directive, BPF
@item .word
The @code{.half} directive produces a 16 bit value.

@cindex @code{word} directive, BPF
@item .word
The @code{.word} directive produces a 32 bit value.

@cindex @code{dword} directive, BPF
@item .dword
The @code{.dword} directive produces a 64 bit value.
@end table

@node BPF Opcodes
@section Opcodes

@cindex BPF opcodes
@cindex opcodes for BPF
In the instruction descriptions below the following field descriptors
are used:

@table @code
@item %d
Destination general-purpose register whose role is to be destination
of an operation.
@item %s
Source general-purpose register whose role is to be the source of an
operation.
@item disp16
16-bit signed PC-relative offset, measured in number of 64-bit words,
minus one.
@item disp32
32-bit signed PC-relative offset, measured in number of 64-bit words,
minus one.
@item offset16
Signed 16-bit immediate.
@item imm32
Signed 32-bit immediate.
@item imm64
Signed 64-bit immediate.
@end table

@subsubsection Arithmetic instructions

The destination register in these instructions act like an
accumulator.

@table @code
@item add %d, (%s|imm32)
64-bit arithmetic addition.
@item sub %d, (%s|imm32)
64-bit arithmetic subtraction.
@item mul %d, (%s|imm32)
64-bit arithmetic multiplication.
@item div %d, (%s|imm32)
64-bit arithmetic integer division.
@item mod %d, (%s|imm32)
64-bit integer remainder.
@item and %d, (%s|imm32)
64-bit bit-wise ``and'' operation.
@item or %d, (%s|imm32)
64-bit bit-wise ``or'' operation.
@item xor %d, (%s|imm32)
64-bit bit-wise exclusive-or operation.
@item lsh %d, (%s|imm32)
64-bit left shift, by @code{%s} or @code{imm32} bits.
@item rsh %d, (%s|imm32)
64-bit right logical shift, by @code{%s} or @code{imm32} bits.
@item arsh %d, (%s|imm32)
64-bit right arithmetic shift, by @code{%s} or @code{imm32} bits.
@item neg %d
64-bit arithmetic negation.
@item mov %d, (%s|imm32)
Move the 64-bit value of @code{%s} in @code{%d}, or load @code{imm32}
in @code{%d}.
@end table

@subsubsection 32-bit arithmetic instructions

The destination register in these instructions act as an accumulator.

@table @code
@item add32 %d, (%s|imm32)
32-bit arithmetic addition.
@item sub32 %d, (%s|imm32)
32-bit arithmetic subtraction.
@item mul32 %d, (%s|imm32)
32-bit arithmetic multiplication.
@item div32 %d, (%s|imm32)
32-bit arithmetic integer division.
@item mod32 %d, (%s|imm32)
32-bit integer remainder.
@item and32 %d, (%s|imm32)
32-bit bit-wise ``and'' operation.
@item or32 %d, (%s|imm32)
32-bit bit-wise ``or'' operation.
@item xor32 %d, (%s|imm32)
32-bit bit-wise exclusive-or operation.
@item lsh32 %d, (%s|imm32)
32-bit left shift, by @code{%s} or @code{imm32} bits.
@item rsh32 %d, (%s|imm32)
32-bit right logical shift, by @code{%s} or @code{imm32} bits.
@item arsh32 %d, (%s|imm32)
32-bit right arithmetic shift, by @code{%s} or @code{imm32} bits.
@item neg32 %d
32-bit arithmetic negation.
@item mov32 %d, (%s|imm32)
Move the 32-bit value of @code{%s} in @code{%d}, or load @code{imm32}
in @code{%d}.
@end table

@subsubsection Endianness conversion instructions

@table @code
@item endle %d, (8|16|32)
Convert the 8-bit, 16-bit or 32-bit value in @code{%d} to
little-endian.
@item endbe %d, (8|16|32)
Convert the 8-bit, 16-bit or 32-bit value in @code{%d} to big-endian.
@end table

@subsubsection 64-bit load and pseudo maps

@table @code
@item lddw %d, imm64
Load the given signed 64-bit immediate, or pseudo map descriptor, to
the destination register @code{%d}.
@item lddw %d, %map_fd(N)
Load the address of the given pseudo map fd @emph{N} to the
destination register @code{%d}.
@end table

@subsubsection Load instructions for socket filters

The following instructions are intended to be used in socket filters,
and are therefore not general-purpose: they make assumptions on the
contents of several registers.  See the file
@file{Documentation/networking/filter.txt} in the Linux kernel source
tree for more information.

Absolute loads:

@table @code
@item ldabsdw imm32
Absolute 64-bit load.
@item ldabsw imm32
Absolute 32-bit load.
@item ldabsh imm32
Absolute 16-bit load.
@item ldabsb imm32
Absolute 8-bit load.
@end table

Indirect loads:

@table @code
@item ldinddw %s, imm32
Indirect 64-bit load.
@item ldindw %s, imm32
Indirect 32-bit load.
@item ldindh %s, imm32
Indirect 16-bit load.
@item ldindb %s, imm32
Indirect 8-bit load.
@end table

@subsubsection Generic load/store instructions

General-purpose load and store instructions are provided for several
word sizes.

Load to register instructions:

@table @code
@item ldxdw %d, [%s+offset16]
Generic 64-bit load.
@item ldxw %d, [%s+offset16]
Generic 32-bit load.
@item ldxh %d, [%s+offset16]
Generic 16-bit load.
@item ldxb %d, [%s+offset16]
Generic 8-bit load.
@end table

Store from register instructions:

@table @code
@item stxdw [%d+offset16], %s
Generic 64-bit store.
@item stxw [%d+offset16], %s
Generic 32-bit store.
@item stxh [%d+offset16], %s
Generic 16-bit store.
@item stxb [%d+offset16], %s
Generic 8-bit store.
@end table

Store from immediates instructions:

@table @code
@item stddw [%d+offset16], imm32
Store immediate as 64-bit.
@item stdw [%d+offset16], imm32
Store immediate as 32-bit.
@item stdh [%d+offset16], imm32
Store immediate as 16-bit.
@item stdb [%d+offset16], imm32
Store immediate as 8-bit.
@end table

@subsubsection Jump instructions

eBPF provides the following compare-and-jump instructions, which
compare the values of the two given registers, or the values of a
register and an immediate, and perform a branch in case the comparison
holds true.

@table @code
@item ja %d,(%s|imm32),disp16
Jump-always.
@item jeq %d,(%s|imm32),disp16
Jump if equal.
@item jgt %d,(%s|imm32),disp16
Jump if greater.
@item jge %d,(%s|imm32),disp16
Jump if greater or equal.
@item jlt %d,(%s|imm32),disp16
Jump if lesser.
@item jle %d,(%s|imm32),disp16
Jump if lesser or equal.
@item jset %d,(%s|imm32),disp16
Jump if signed equal.
@item jne %d,(%s|imm32),disp16
Jump if not equal.
@item jsgt %d,(%s|imm32),disp16
Jump if signed greater.
@item jsge %d,(%s|imm32),disp16
Jump if signed greater or equal.
@item jslt %d,(%s|imm32),disp16
Jump if signed lesser.
@item jsle %d,(%s|imm32),disp16
Jump if signed lesser or equal.
@end table

A call instruction is provided in order to perform calls to other eBPF
functions, or to external kernel helpers:

@table @code
@item call (disp32|imm32)
Jump and link to the offset @emph{disp32}, or to the kernel helper
function identified by @emph{imm32}.
@end table

Finally:

@table @code
@item exit
Terminate the eBPF program.
@end table

@subsubsection Atomic instructions

Atomic exchange-and-add instructions are provided in two flavors: one
for swapping 64-bit quantities and another for 32-bit quantities.

@table @code
@item xadddw [%d+offset16],%s
Exchange-and-add a 64-bit value at the specified location.
@item xaddw [%d+offset16],%s
Exchange-and-add a 32-bit value at the specified location.
@end table