Re: [PATCH bpf-next 2/2] bpf,riscv: Implement bpf_addr_space_cast instruction

[Alexei Starovoitov]

On Sat, Mar 23, 2024 at 8:47 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
>
> LLVM generates bpf_addr_space_cast instruction while translating
> pointers between native (zero) address space and
> __attribute__((address_space(N))). The addr_space=0 is reserved as
> bpf_arena address space.
>
> rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
> converted to normal 32-bit move: wX = wY
>
> rY = addr_space_cast(rX, 1, 0) has to be converted by JIT:
>
> Here I explain using symbolic language what the JIT is supposed to do:
> We have:
>         src = [src_upper32][src_lower32] // 64 bit src kernel pointer
>         uvm = [uvm_upper32][uvm_lower32] // 64 bit user_vm_start

This is a bit misleading.
src_lower32 are always equal to uvm_lower32
and src_upper32 are either zero or uvm_upper32.

Hence most of the time llvm doesn't generate this insn,
since it knows that upper 32 bit are uvm_upper32.

> The JIT has to make the dst reg like following
>         dst = [uvm_upper32][src_lower32] // if src_lower32 != 0
>         dst = [00000000000][00000000000] // if src_lower32 == 0
>
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
>  arch/riscv/net/bpf_jit.h        |  1 +
>  arch/riscv/net/bpf_jit_comp64.c | 15 +++++++++++++++
>  arch/riscv/net/bpf_jit_core.c   |  1 +
>  3 files changed, 17 insertions(+)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index 8a47da08dd9c..5fc374ed98ea 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -82,6 +82,7 @@ struct rv_jit_context {
>         unsigned long flags;
>         int stack_size;
>         u64 arena_vm_start;
> +       u64 user_vm_start;
>  };
>
>  /* Convert from ninsns to bytes. */
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index f51b832eafb6..3c389e75cb96 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1083,6 +1083,16 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>         /* dst = src */
>         case BPF_ALU | BPF_MOV | BPF_X:
>         case BPF_ALU64 | BPF_MOV | BPF_X:
> +               if (BPF_CLASS(insn->code) == BPF_ALU64 && insn->off == BPF_ADDR_SPACE_CAST &&
> +                   insn->imm == 1U << 16) {

Let's add a generic helper like insn_is_zext(),
call it insn_is_cast_user() ?
and use it in all JIT-s ?
I should have added it right away when I did x86 part. Sorry.

And a comment next to the helper that addr space cast 0->1
is for converting bpf arena pointers to user vma.
Hence the name.

Same comments for arm64 JIT arena support.

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