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Subject: Re: [PATCH v2 bpf-next 00/13] Atomics for eBPF
To:     Brendan Jackman <jackmanb@google.com>, <bpf@vger.kernel.org>
CC:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>
References: <20201127175738.1085417-1-jackmanb@google.com>
From:   Yonghong Song <yhs@fb.com>
Message-ID: <829353e6-d90a-a91a-418b-3c2556061cda@fb.com>
Date:   Fri, 27 Nov 2020 21:53:05 -0800
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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
>