Re: [PATCH 3/5] bpf ppc64: Add instructions for atomic_[cmp]xchg

[Russell Currey <ruscur@russell.cc>]

On Thu, 2022-05-12 at 13:15 +0530, Hari Bathini wrote:
> This adds two atomic opcodes BPF_XCHG and BPF_CMPXCHG on ppc64, both
> of which include the BPF_FETCH flag.  The kernel's atomic_cmpxchg
> operation fundamentally has 3 operands, but we only have two register
> fields. Therefore the operand we compare against (the kernel's API
> calls it 'old') is hard-coded to be BPF_REG_R0. Also, kernel's
> atomic_cmpxchg returns the previous value at dst_reg + off. JIT the
> same for BPF too with return value put in BPF_REG_0.
> 
>   BPF_REG_R0 = atomic_cmpxchg(dst_reg + off, BPF_REG_R0, src_reg);
> 
> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
> ---
>  arch/powerpc/net/bpf_jit_comp64.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c
> b/arch/powerpc/net/bpf_jit_comp64.c
> index 504fa459f9f3..df9e20b22ccb 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -783,6 +783,9 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32
> *image, struct codegen_context *
>                  */
>                 case BPF_STX | BPF_ATOMIC | BPF_W:
>                 case BPF_STX | BPF_ATOMIC | BPF_DW:
> +                       u32 save_reg = tmp2_reg;
> +                       u32 ret_reg = src_reg;

Hi Hari,

Some compilers[0][1] don't like these late declarations after case
labels:

   arch/powerpc/net/bpf_jit_comp64.c: In function ‘bpf_jit_build_body’:
   arch/powerpc/net/bpf_jit_comp64.c:781:4: error: a label can only be
   part of a statement and a declaration is not a statement
       u32 save_reg = tmp2_reg;
       ^~~
   arch/powerpc/net/bpf_jit_comp64.c:782:4: error: expected expression
   before ‘u32’
       u32 ret_reg = src_reg;
       ^~~
   arch/powerpc/net/bpf_jit_comp64.c:819:5: error: ‘ret_reg’ undeclared
   (first use in this function); did you mean ‘dst_reg’?
        ret_reg = bpf_to_ppc(BPF_REG_0);
   
Adding a semicolon fixes the first issue, i.e.

   case BPF_STX | BPF_ATOMIC | BPF_DW: ;
   
but then it just complains about mixed declarations and code instead.

So you should declare save_reg and ret_reg at the beginning of the for
loop like the rest of the variables.

- Russell

[0]: gcc 5.5.0
https://github.com/ruscur/linux-ci/runs/6418546193?check_suite_focus=true#step:4:122
[1]: clang 12.0
https://github.com/ruscur/linux-ci/runs/6418545338?check_suite_focus=true#step:4:117

> +
>                         /* Get offset into TMP_REG_1 */
>                         EMIT(PPC_RAW_LI(tmp1_reg, off));
>                         tmp_idx = ctx->idx * 4;
> @@ -813,6 +816,24 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32
> *image, struct codegen_context *
>                         case BPF_XOR | BPF_FETCH:
>                                 EMIT(PPC_RAW_XOR(tmp2_reg, tmp2_reg,
> src_reg));
>                                 break;
> +                       case BPF_CMPXCHG:
> +                               /*
> +                                * Return old value in BPF_REG_0 for
> BPF_CMPXCHG &
> +                                * in src_reg for other cases.
> +                                */
> +                               ret_reg = bpf_to_ppc(BPF_REG_0);
> +
> +                               /* Compare with old value in BPF_R0
> */
> +                               if (size == BPF_DW)
> +                                       EMIT(PPC_RAW_CMPD(bpf_to_ppc(
> BPF_REG_0), tmp2_reg));
> +                               else
> +                                       EMIT(PPC_RAW_CMPW(bpf_to_ppc(
> BPF_REG_0), tmp2_reg));
> +                               /* Don't set if different from old
> value */
> +                               PPC_BCC_SHORT(COND_NE, (ctx->idx + 3)
> * 4);
> +                               fallthrough;
> +                       case BPF_XCHG:
> +                               save_reg = src_reg;
> +                               break;
>                         default:
>                                 pr_err_ratelimited(
>                                         "eBPF filter atomic op code
> %02x (@%d) unsupported\n",
> @@ -822,15 +843,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32
> *image, struct codegen_context *
>  
>                         /* store new value */
>                         if (size == BPF_DW)
> -                               EMIT(PPC_RAW_STDCX(tmp2_reg,
> tmp1_reg, dst_reg));
> +                               EMIT(PPC_RAW_STDCX(save_reg,
> tmp1_reg, dst_reg));
>                         else
> -                               EMIT(PPC_RAW_STWCX(tmp2_reg,
> tmp1_reg, dst_reg));
> +                               EMIT(PPC_RAW_STWCX(save_reg,
> tmp1_reg, dst_reg));
>                         /* we're done if this succeeded */
>                         PPC_BCC_SHORT(COND_NE, tmp_idx);
>  
> -                       /* For the BPF_FETCH variant, get old value
> into src_reg */
>                         if (imm & BPF_FETCH)
> -                               EMIT(PPC_RAW_MR(src_reg, _R0));
> +                               EMIT(PPC_RAW_MR(ret_reg, _R0));
>                         break;
>  
>                 /*