On Tue, Jul 21, 2020 at 12:27 AM Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
>
> On Tue, 21 Jul 2020 at 04:52, Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> Acked-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
Applied. Thanks
