netdev - Re: [PATCH bpf-next 01/13] bpf: xor of a/x in cbpf can be done

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Date:   Mon, 29 Jan 2018 00:28:57 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     ast@...nel.org, Daniel Borkmann <daniel@...earbox.net>
Cc:     netdev@...r.kernel.org, Sandipan Das <sandipan@...ux.vnet.ibm.com>
Subject: Re: [PATCH bpf-next 01/13] bpf: xor of a/x in cbpf can be done

in 32 bit alu

Daniel Borkmann wrote:
> Very minor optimization; saves 1 byte per program in x86_64
> JIT in cBPF prologue.

... but increases program size by 4 bytes on ppc64 :(
In general, this is an area I've been wanting to spend some time on.  
Powerpc doesn't have 32-bit sub-registers, so we need to emit an 
additional instruction to clear the higher 32-bits for all 32-bit 
operations. I need to look at the performance impact.

- Naveen

> 
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  net/core/filter.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 18da42a..cba2f73 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -401,8 +401,8 @@ static int bpf_convert_filter(struct sock_filter *prog, int len,
>  		/* Classic BPF expects A and X to be reset first. These need
>  		 * to be guaranteed to be the first two instructions.
>  		 */
> -		*new_insn++ = BPF_ALU64_REG(BPF_XOR, BPF_REG_A, BPF_REG_A);
> -		*new_insn++ = BPF_ALU64_REG(BPF_XOR, BPF_REG_X, BPF_REG_X);
> +		*new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_A, BPF_REG_A);
> +		*new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_X, BPF_REG_X);
>  
>  		/* All programs must keep CTX in callee saved BPF_REG_CTX.
>  		 * In eBPF case it's done by the compiler, here we need to
> -- 
> 2.9.5
> 
> 
>