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Date:   Tue, 21 Jul 2020 12:42:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Luke Nelson <lukenels@...washington.edu>,
        bpf <bpf@...r.kernel.org>, Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>,
        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>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Netdev <netdev@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 0/3] bpf, riscv: Add compressed instructions
 to rv64 JIT

On Tue, Jul 21, 2020 at 12:27 AM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> On Tue, 21 Jul 2020 at 04:52, Luke Nelson <lukenels@...washington.edu> wrote:
> >
> > This patch series enables using compressed riscv (RVC) instructions
> > in the rv64 BPF JIT.
> >
> > 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
> >
> > RFC -> v1:
> >   - From Björn Töpel:
> >     * Changed RVOFF macro to static inline "ninsns_rvoff".
> >     * Changed return type of rvc_ functions from u32 to u16.
> >     * Changed sizeof(u16) to sizeof(*ctx->insns).
> >   * Factored unsigned immediate checks into helper functions
> >     (is_8b_uint, etc.)
> >   * Changed to use IS_ENABLED instead of #ifdef to check if RVC is
> >     enabled.
> >   * Changed type of immediate arguments to rvc_* encoding to u32
> >     to avoid issues from promotion of u16 to signed int.
> >   * Cleaned up RVC checks in emit_{addi,slli,srli,srai}.
> >     + Wrapped lines at 100 instead of 80 columns for increased clarity.
> >         + Move !imm checks into each branch instead of checking
> >           separately.
> >         + Strengthed checks for c.{slli,srli,srai} to check that
> >           imm < XLEN. Otherwise, imm could be non-zero but the lower
> >           XLEN bits could all be zero, leading to invalid RVC encoding.
> >   * Changed emit_imm to sign-extend the 12-bit value in "lower"
> >     + The immediate checks for emit_{addiw,li,addi} use signed
> >           comparisons, so this enables the RVC variants to be used
> >           more often (e.g., if val == -1, then lower should be -1
> >           as opposed to 4095).
> >
>
> Finally RVC support! Thank you!
>
> For the series:
> Reviewed-by: Björn Töpel <bjorn.topel@...il.com>
> Acked-by: Björn Töpel <bjorn.topel@...il.com>

Applied. Thanks

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