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Date: Fri, 27 Nov 2020 21:53:05 -0800
From: Yonghong Song <yhs@...com>
To: Brendan Jackman <jackmanb@...gle.com>, <bpf@...r.kernel.org>
CC: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
KP Singh <kpsingh@...omium.org>,
Florent Revest <revest@...omium.org>,
<linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 00/13] Atomics for eBPF
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@google.com/
>
> [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
>
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