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Message-ID: <CAJ+HfNg_qV=umB9T1U9vRo2pUpmUFfBN44WpAOfMoi75Ymh2dw@mail.gmail.com>
Date: Wed, 15 Jul 2020 19:56:06 +0200
From: Björn Töpel <bjorn.topel@...il.com>
To: Luke Nelson <lukenels@...washington.edu>
Cc: bpf <bpf@...r.kernel.org>, Luke Nelson <luke.r.nels@...il.com>,
Xi Wang <xi.wang@...il.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
Andrii Nakryiko <andriin@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...omium.org>,
Netdev <netdev@...r.kernel.org>,
linux-riscv <linux-riscv@...ts.infradead.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf, riscv: Add compressed instructions
to rv64 JIT
On Mon, 13 Jul 2020 at 20:37, Luke Nelson <lukenels@...washington.edu> wrote:
>
> This patch series enables using compressed riscv (RVC) instructions
> in the rv64 BPF JIT.
>
First of all; Really nice work. I like this, and it makes the code
easier to read as well (e.g. emit_mv). I'm a bit curious why you only
did it for RV64, and not RV32? I have some minor comments on the
patches. I strongly encourage you to submit this as a proper (non-RFC)
set for bpf-next.
Cheers,
Björn
> RVC is a standard riscv extension that adds a set of compressed,
> 2-byte instructions that can replace some regular 4-byte instructions
> for improved code density.
>
> This series first modifies the JIT to support using 2-byte instructions
> (e.g., in jump offset computations), then adds RVC encoding and
> helper functions, and finally uses the helper functions to optimize
> the rv64 JIT.
>
> I used our formal verification framework, Serval, to verify the
> correctness of the RVC encodings and their uses in the rv64 JIT.
>
> The JIT continues to pass all tests in lib/test_bpf.c, and introduces
> no new failures to test_verifier; both with and without RVC being enabled.
>
> The following are examples of the JITed code for the verifier selftest
> "direct packet read test#3 for CGROUP_SKB OK", without and with RVC
> enabled, respectively. The former uses 178 bytes, and the latter uses 112,
> for a ~37% reduction in code size for this example.
>
> Without RVC:
>
> 0: 02000813 addi a6,zero,32
> 4: fd010113 addi sp,sp,-48
> 8: 02813423 sd s0,40(sp)
> c: 02913023 sd s1,32(sp)
> 10: 01213c23 sd s2,24(sp)
> 14: 01313823 sd s3,16(sp)
> 18: 01413423 sd s4,8(sp)
> 1c: 03010413 addi s0,sp,48
> 20: 03056683 lwu a3,48(a0)
> 24: 02069693 slli a3,a3,0x20
> 28: 0206d693 srli a3,a3,0x20
> 2c: 03456703 lwu a4,52(a0)
> 30: 02071713 slli a4,a4,0x20
> 34: 02075713 srli a4,a4,0x20
> 38: 03856483 lwu s1,56(a0)
> 3c: 02049493 slli s1,s1,0x20
> 40: 0204d493 srli s1,s1,0x20
> 44: 03c56903 lwu s2,60(a0)
> 48: 02091913 slli s2,s2,0x20
> 4c: 02095913 srli s2,s2,0x20
> 50: 04056983 lwu s3,64(a0)
> 54: 02099993 slli s3,s3,0x20
> 58: 0209d993 srli s3,s3,0x20
> 5c: 09056a03 lwu s4,144(a0)
> 60: 020a1a13 slli s4,s4,0x20
> 64: 020a5a13 srli s4,s4,0x20
> 68: 00900313 addi t1,zero,9
> 6c: 006a7463 bgeu s4,t1,0x74
> 70: 00000a13 addi s4,zero,0
> 74: 02d52823 sw a3,48(a0)
> 78: 02e52a23 sw a4,52(a0)
> 7c: 02952c23 sw s1,56(a0)
> 80: 03252e23 sw s2,60(a0)
> 84: 05352023 sw s3,64(a0)
> 88: 00000793 addi a5,zero,0
> 8c: 02813403 ld s0,40(sp)
> 90: 02013483 ld s1,32(sp)
> 94: 01813903 ld s2,24(sp)
> 98: 01013983 ld s3,16(sp)
> 9c: 00813a03 ld s4,8(sp)
> a0: 03010113 addi sp,sp,48
> a4: 00078513 addi a0,a5,0
> a8: 00008067 jalr zero,0(ra)
>
> With RVC:
>
> 0: 02000813 addi a6,zero,32
> 4: 7179 c.addi16sp sp,-48
> 6: f422 c.sdsp s0,40(sp)
> 8: f026 c.sdsp s1,32(sp)
> a: ec4a c.sdsp s2,24(sp)
> c: e84e c.sdsp s3,16(sp)
> e: e452 c.sdsp s4,8(sp)
> 10: 1800 c.addi4spn s0,sp,48
> 12: 03056683 lwu a3,48(a0)
> 16: 1682 c.slli a3,0x20
> 18: 9281 c.srli a3,0x20
> 1a: 03456703 lwu a4,52(a0)
> 1e: 1702 c.slli a4,0x20
> 20: 9301 c.srli a4,0x20
> 22: 03856483 lwu s1,56(a0)
> 26: 1482 c.slli s1,0x20
> 28: 9081 c.srli s1,0x20
> 2a: 03c56903 lwu s2,60(a0)
> 2e: 1902 c.slli s2,0x20
> 30: 02095913 srli s2,s2,0x20
> 34: 04056983 lwu s3,64(a0)
> 38: 1982 c.slli s3,0x20
> 3a: 0209d993 srli s3,s3,0x20
> 3e: 09056a03 lwu s4,144(a0)
> 42: 1a02 c.slli s4,0x20
> 44: 020a5a13 srli s4,s4,0x20
> 48: 4325 c.li t1,9
> 4a: 006a7363 bgeu s4,t1,0x50
> 4e: 4a01 c.li s4,0
> 50: d914 c.sw a3,48(a0)
> 52: d958 c.sw a4,52(a0)
> 54: dd04 c.sw s1,56(a0)
> 56: 03252e23 sw s2,60(a0)
> 5a: 05352023 sw s3,64(a0)
> 5e: 4781 c.li a5,0
> 60: 7422 c.ldsp s0,40(sp)
> 62: 7482 c.ldsp s1,32(sp)
> 64: 6962 c.ldsp s2,24(sp)
> 66: 69c2 c.ldsp s3,16(sp)
> 68: 6a22 c.ldsp s4,8(sp)
> 6a: 6145 c.addi16sp sp,48
> 6c: 853e c.mv a0,a5
> 6e: 8082 c.jr ra
>
> Luke Nelson (3):
> bpf, riscv: Modify JIT ctx to support compressed instructions
> bpf, riscv: Add encodings for compressed instructions
> bpf, riscv: Use compressed instructions in the rv64 JIT
>
> arch/riscv/net/bpf_jit.h | 495 +++++++++++++++++++++++++++++++-
> arch/riscv/net/bpf_jit_comp32.c | 14 +-
> arch/riscv/net/bpf_jit_comp64.c | 287 +++++++++---------
> arch/riscv/net/bpf_jit_core.c | 6 +-
> 4 files changed, 650 insertions(+), 152 deletions(-)
>
> --
> 2.25.1
>
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