netdev - Re: [PATCH bpf-next] bpf, x86: Remove unused cnt increase from EMIT macro

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Date:   Thu, 17 Jun 2021 13:04:08 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next] bpf, x86: Remove unused cnt increase from EMIT
 macro

On 6/16/21 3:34 PM, Jiri Olsa wrote:
> Removing unused cnt increase from EMIT macro together
> with cnt declarations. This was introduced in commit [1]
> to ensure proper code generation. But that code was
> removed in commit [2] and this extra code was left in.
> 
> [1] b52f00e6a715 ("x86: bpf_jit: implement bpf_tail_call() helper")
> [2] ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
> 
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> ---
>   arch/x86/net/bpf_jit_comp.c | 39 ++++++++++---------------------------
>   1 file changed, 10 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2a2e290fa5d8..19715542cd9c 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -31,7 +31,7 @@ static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
>   }
>   
>   #define EMIT(bytes, len) \
> -	do { prog = emit_code(prog, bytes, len); cnt += len; } while (0)
> +	do { prog = emit_code(prog, bytes, len); } while (0)
>   
>   #define EMIT1(b1)		EMIT(b1, 1)
>   #define EMIT2(b1, b2)		EMIT((b1) + ((b2) << 8), 2)
> @@ -239,7 +239,6 @@ struct jit_context {
>   static void push_callee_regs(u8 **pprog, bool *callee_regs_used)
>   {
>   	u8 *prog = *pprog;
> -	int cnt = 0;
>   
>   	if (callee_regs_used[0])
>   		EMIT1(0x53);         /* push rbx */
> @@ -255,7 +254,6 @@ static void push_callee_regs(u8 **pprog, bool *callee_regs_used)
>   static void pop_callee_regs(u8 **pprog, bool *callee_regs_used)
>   {
>   	u8 *prog = *pprog;
> -	int cnt = 0;
>   
>   	if (callee_regs_used[3])
>   		EMIT2(0x41, 0x5F);   /* pop r15 */
> @@ -303,7 +301,6 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,

nit: In emit_prologue() we also have cnt that we could just replace with X86_PATCH_SIZE
directly as well.

>   static int emit_patch(u8 **pprog, void *func, void *ip, u8 opcode)
>   {
>   	u8 *prog = *pprog;

Otherwise, lgtm.

Thanks,
Daniel