On 11/27/20 9:57 AM, Brendan Jackman wrote:
Status of the patches
=====================
Thanks for the reviews! Differences from v1->v2 [1]:
* Fixed mistakes in the netronome driver
* Addd sub, add, or, xor operations
* The above led to some refactors to keep things readable. (Maybe I
should have just waited until I'd implemented these before starting
the review...)
* Replaced BPF_[CMP]SET | BPF_FETCH with just BPF_[CMP]XCHG, which
include the BPF_FETCH flag
* Added a bit of documentation. Suggestions welcome for more places
to dump this info...
The prog_test that's added depends on Clang/LLVM features added by
Yonghong in https://reviews.llvm.org/D72184
This only includes a JIT implementation for x86_64 - I don't plan to
implement JIT support myself for other architectures.
Operations
==========
This patchset adds atomic operations to the eBPF instruction set. The
use-case that motivated this work was a trivial and efficient way to
generate globally-unique cookies in BPF progs, but I think it's
obvious that these features are pretty widely applicable. The
instructions that are added here can be summarised with this list of
kernel operations:
* atomic[64]_[fetch_]add
* atomic[64]_[fetch_]sub
* atomic[64]_[fetch_]and
* atomic[64]_[fetch_]or
* atomic[64]_[fetch_]xor
* atomic[64]_xchg
* atomic[64]_cmpxchg
Thanks. Overall looks good to me but I did not check carefully
on jit part as I am not an expert in x64 assembly...
This patch also introduced atomic[64]_{sub,and,or,xor}, similar to
xadd. I am not sure whether it is necessary. For one thing,
users can just use atomic[64]_fetch_{sub,and,or,xor} to ignore
return value and they will achieve the same result, right?
From llvm side, there is no ready-to-use gcc builtin matching
atomic[64]_{sub,and,or,xor} which does not have return values.
If we go this route, we will need to invent additional bpf
specific builtins.
The following are left out of scope for this effort:
* 16 and 8 bit operations
* Explicit memory barriers
Encoding
========
I originally planned to add new values for bpf_insn.opcode. This was
rather unpleasant: the opcode space has holes in it but no entire
instruction classes[2]. Yonghong Song had a better idea: use the
immediate field of the existing STX XADD instruction to encode the
operation. This works nicely, without breaking existing programs,
because the immediate field is currently reserved-must-be-zero, and
extra-nicely because BPF_ADD happens to be zero.
Note that this of course makes immediate-source atomic operations
impossible. It's hard to imagine a measurable speedup from such
instructions, and if it existed it would certainly not benefit x86,
which has no support for them.
The BPF_OP opcode fields are re-used in the immediate, and an
additional flag BPF_FETCH is used to mark instructions that should
fetch a pre-modification value from memory.
So, BPF_XADD is now called BPF_ATOMIC (the old name is kept to avoid
breaking userspace builds), and where we previously had .imm = 0, we
now have .imm = BPF_ADD (which is 0).
Operands
========
Reg-source eBPF instructions only have two operands, while these
atomic operations have up to four. To avoid needing to encode
additional operands, then:
- One of the input registers is re-used as an output register
(e.g. atomic_fetch_add both reads from and writes to the source
register).
- Where necessary (i.e. for cmpxchg) , R0 is "hard-coded" as one of
the operands.
This approach also allows the new eBPF instructions to map directly
to single x86 instructions.
[1] Previous patchset:
https://lore.kernel.org/bpf/20201123173202.1335708-1-jackmanb@xxxxxxxxxx/
[2] Visualisation of eBPF opcode space:
https://gist.github.com/bjackman/00fdad2d5dfff601c1918bc29b16e778
Brendan Jackman (13):
bpf: x86: Factor out emission of ModR/M for *(reg + off)
bpf: x86: Factor out emission of REX byte
bpf: x86: Factor out function to emit NEG
bpf: x86: Factor out a lookup table for some ALU opcodes
bpf: Rename BPF_XADD and prepare to encode other atomics in .imm
bpf: Move BPF_STX reserved field check into BPF_STX verifier code
bpf: Add BPF_FETCH field / create atomic_fetch_add instruction
bpf: Add instructions for atomic_[cmp]xchg
bpf: Pull out a macro for interpreting atomic ALU operations
bpf: Add instructions for atomic[64]_[fetch_]sub
bpf: Add bitwise atomic instructions
bpf: Add tests for new BPF atomic operations
bpf: Document new atomic instructions
Documentation/networking/filter.rst | 57 ++-
arch/arm/net/bpf_jit_32.c | 7 +-
arch/arm64/net/bpf_jit_comp.c | 16 +-
arch/mips/net/ebpf_jit.c | 11 +-
arch/powerpc/net/bpf_jit_comp64.c | 25 +-
arch/riscv/net/bpf_jit_comp32.c | 20 +-
arch/riscv/net/bpf_jit_comp64.c | 16 +-
arch/s390/net/bpf_jit_comp.c | 27 +-
arch/sparc/net/bpf_jit_comp_64.c | 17 +-
arch/x86/net/bpf_jit_comp.c | 252 ++++++++++----
arch/x86/net/bpf_jit_comp32.c | 6 +-
drivers/net/ethernet/netronome/nfp/bpf/jit.c | 14 +-
drivers/net/ethernet/netronome/nfp/bpf/main.h | 4 +-
.../net/ethernet/netronome/nfp/bpf/verifier.c | 15 +-
include/linux/filter.h | 117 ++++++-
include/uapi/linux/bpf.h | 8 +-
kernel/bpf/core.c | 67 +++-
kernel/bpf/disasm.c | 41 ++-
kernel/bpf/verifier.c | 77 +++-
lib/test_bpf.c | 2 +-
samples/bpf/bpf_insn.h | 4 +-
samples/bpf/sock_example.c | 2 +-
samples/bpf/test_cgrp2_attach.c | 4 +-
tools/include/linux/filter.h | 117 ++++++-
tools/include/uapi/linux/bpf.h | 8 +-
tools/testing/selftests/bpf/Makefile | 12 +-
.../selftests/bpf/prog_tests/atomics_test.c | 329 ++++++++++++++++++
.../bpf/prog_tests/cgroup_attach_multi.c | 4 +-
.../selftests/bpf/progs/atomics_test.c | 124 +++++++
.../selftests/bpf/verifier/atomic_and.c | 77 ++++
.../selftests/bpf/verifier/atomic_cmpxchg.c | 96 +++++
.../selftests/bpf/verifier/atomic_fetch_add.c | 106 ++++++
.../selftests/bpf/verifier/atomic_or.c | 77 ++++
.../selftests/bpf/verifier/atomic_sub.c | 44 +++
.../selftests/bpf/verifier/atomic_xchg.c | 46 +++
.../selftests/bpf/verifier/atomic_xor.c | 77 ++++
tools/testing/selftests/bpf/verifier/ctx.c | 7 +-
.../testing/selftests/bpf/verifier/leak_ptr.c | 4 +-
tools/testing/selftests/bpf/verifier/unpriv.c | 3 +-
tools/testing/selftests/bpf/verifier/xadd.c | 2 +-
40 files changed, 1754 insertions(+), 188 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/atomics_test.c
create mode 100644 tools/testing/selftests/bpf/progs/atomics_test.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_and.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_cmpxchg.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_fetch_add.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_or.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_sub.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xchg.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xor.c
--
2.29.2.454.gaff20da3a2-goog
