| Message ID | 20201203160245.1014867-1-jackmanb@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Atomics for eBPF | expand |
On Thu, Dec 03, 2020 at 04:02:31PM +0000, Brendan Jackman wrote: [...] > [1] Previous patchset: > https://lore.kernel.org/bpf/20201123173202.1335708-1-jackmanb@google.com/ Sorry, bogus link. That's v1, here's v2: https://lore.kernel.org/bpf/20201127175738.1085417-1-jackmanb@google.com/
On 12/3/20 8:02 AM, Brendan Jackman wrote: > Status of the patches > ===================== > > Thanks for the reviews! 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 https://reviews.llvm.org/D72184 Just let you know that the above patch has been merged into llvm-project trunk, so you do not manually apply it any more. > > 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 patchset: > https://lore.kernel.org/bpf/20201123173202.1335708-1-jackmanb@google.com/ > > [2] Visualisation of eBPF opcode space: > https://gist.github.com/bjackman/00fdad2d5dfff601c1918bc29b16e778 > [...]
Status of the patches ===================== Thanks for the reviews! 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 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 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 (14): 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 bitwise atomic instructions tools build: Implement feature check for BPF atomics in Clang bpf: Pull tools/build/feature biz into selftests Makefile 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 | 241 +++++++++++----- 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 | 97 ++++++- include/uapi/linux/bpf.h | 8 +- kernel/bpf/core.c | 66 ++++- kernel/bpf/disasm.c | 43 ++- kernel/bpf/verifier.c | 75 +++-- 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/build/feature/Makefile | 4 + tools/build/feature/test-clang-bpf-atomics.c | 9 + tools/include/linux/filter.h | 97 ++++++- tools/include/uapi/linux/bpf.h | 8 +- tools/testing/selftests/bpf/.gitignore | 1 + tools/testing/selftests/bpf/Makefile | 42 +++ .../selftests/bpf/prog_tests/atomics_test.c | 262 ++++++++++++++++++ .../bpf/prog_tests/cgroup_attach_multi.c | 4 +- .../selftests/bpf/progs/atomics_test.c | 154 ++++++++++ .../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 +- .../testing/selftests/bpf/verifier/leak_ptr.c | 4 +- tools/testing/selftests/bpf/verifier/unpriv.c | 3 +- tools/testing/selftests/bpf/verifier/xadd.c | 2 +- 42 files changed, 1666 insertions(+), 186 deletions(-) create mode 100644 tools/build/feature/test-clang-bpf-atomics.c 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_xchg.c create mode 100644 tools/testing/selftests/bpf/verifier/atomic_xor.c base-commit: 97306be45fbe7a02461c3c2a57e666cf662b1aaf -- 2.29.2.454.gaff20da3a2-goog