[PATCH bpf-next v4 00/11] Atomics for eBPF
From: Brendan Jackman
Date: Mon Dec 07 2020 - 11:09:20 EST
Status of the patches
=====================
Thanks for the reviews! Differences from v3->v4 [1]:
* Added one Ack from Yonghong. He acked some other patches but those
have now changed non-trivally so I didn't add those acks.
* Fixups to commit messages.
* Fixed disassembly and comments: first arg to atomic_fetch_* is a
pointer.
* Improved prog_test efficiency. BPF progs are now all loaded in a
single call, then the skeleton is re-used for each subtest.
* Dropped use of tools/build/feature in favour of a one-liner in the
Makefile.
* Dropped the commit that created an emit_neg helper in the x86
JIT. It's not used any more (it wasn't used in v3 either).
* Combined all the different filter.h macros (used to be
BPF_ATOMIC_ADD, BPF_ATOMIC_FETCH_ADD, BPF_ATOMIC_AND, etc) into
just BPF_ATOMIC32 and BPF_ATOMIC64.
* Removed some references to BPF_STX_XADD from tools/, samples/ and
lib/ that I missed before.
Differences from v2->v3 [1]:
* More minor fixes and naming/comment changes
* Dropped atomic subtract: compilers can implement this by preceding
an atomic add with a NEG instruction (which is what the x86 JIT did
under the hood anyway).
* Dropped the use of -mcpu=v4 in the Clang BPF command-line; there is
no longer an architecture version bump. Instead a feature test is
added to Kbuild - it builds a source file to check if Clang
supports BPF atomics.
* Fixed the prog_test so it no longer breaks
test_progs-no_alu32. This requires some ifdef acrobatics to avoid
complicating the prog_tests model where the same userspace code
exercises both the normal and no_alu32 BPF test objects, using the
same skeleton header.
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 commit 286daafd6512 (was
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_]and
* atomic[64]_[fetch_]or
* atomic[64]_xchg
* atomic[64]_cmpxchg
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 iterations:
v1: https://lore.kernel.org/bpf/20201123173202.1335708-1-jackmanb@xxxxxxxxxx/
v2: https://lore.kernel.org/bpf/20201127175738.1085417-1-jackmanb@xxxxxxxxxx/
v3: https://lore.kernel.org/bpf/X8kN7NA7bJC7aLQI@xxxxxxxxxx/
[2] Visualisation of eBPF opcode space:
https://gist.github.com/bjackman/00fdad2d5dfff601c1918bc29b16e778
Brendan Jackman (11):
bpf: x86: Factor out emission of ModR/M for *(reg + off)
bpf: x86: Factor out emission of REX byte
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 bitwise atomic instructions
bpf: Add tests for new BPF atomic operations
bpf: Document new atomic instructions
Documentation/networking/filter.rst | 56 +++-
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 | 217 ++++++++++-----
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 | 118 ++++++++-
include/uapi/linux/bpf.h | 10 +-
kernel/bpf/core.c | 67 ++++-
kernel/bpf/disasm.c | 43 ++-
kernel/bpf/verifier.c | 75 ++++--
lib/test_bpf.c | 14 +-
samples/bpf/bpf_insn.h | 4 +-
samples/bpf/cookie_uid_helper_example.c | 6 +-
samples/bpf/sock_example.c | 2 +-
samples/bpf/test_cgrp2_attach.c | 5 +-
tools/include/linux/filter.h | 127 ++++++++-
tools/include/uapi/linux/bpf.h | 10 +-
tools/testing/selftests/bpf/Makefile | 10 +
.../selftests/bpf/prog_tests/atomics.c | 246 ++++++++++++++++++
.../bpf/prog_tests/cgroup_attach_multi.c | 4 +-
tools/testing/selftests/bpf/progs/atomics.c | 154 +++++++++++
.../selftests/bpf/test_cgroup_storage.c | 2 +-
.../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_xchg.c | 46 ++++
.../selftests/bpf/verifier/atomic_xor.c | 77 ++++++
tools/testing/selftests/bpf/verifier/ctx.c | 7 +-
.../bpf/verifier/direct_packet_access.c | 4 +-
.../testing/selftests/bpf/verifier/leak_ptr.c | 10 +-
.../selftests/bpf/verifier/meta_access.c | 4 +-
tools/testing/selftests/bpf/verifier/unpriv.c | 3 +-
.../bpf/verifier/value_illegal_alu.c | 2 +-
tools/testing/selftests/bpf/verifier/xadd.c | 18 +-
44 files changed, 1665 insertions(+), 210 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/atomics.c
create mode 100644 tools/testing/selftests/bpf/progs/atomics.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_xchg.c
create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xor.c
base-commit: 34da87213d3ddd26643aa83deff7ffc6463da0fc
--
2.29.2.576.ga3fc446d84-goog