Re: [RFC PATCH bpf-next v4 1/4] bpf, x64: Comment tail_call_cnt initialisation

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Sun, Sep 03, 2023 at 11:14:45PM +0800, Leon Hwang wrote:
> Without understanding emit_prologue(), it is really hard to figure out
> where does tail_call_cnt come from, even though searching tail_call_cnt
> in the whole kernel repo.
> 
> By adding these comments, it is a little bit easy to understand

s/easy/easier

> tail_call_cnt initialisation.
> 
> Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>

I was rather thinking about dropping these comments altogether. We should
be trying to provide fixes as small as possible IMHO.

Having this as a separate commit also breaks logistics of this set as
the fix + selftest should be routed towards bpf tree, whereas this
particular commit is rather a bpf-next candidate.

PTAL at https://www.kernel.org/doc/html/latest/bpf/bpf_devel_QA.html

> ---
>  arch/x86/net/bpf_jit_comp.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index a5930042139d3..bcca1c9b9a027 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -303,8 +303,12 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
>  	prog += X86_PATCH_SIZE;
>  	if (!ebpf_from_cbpf) {
>  		if (tail_call_reachable && !is_subprog)
> +			/* When it's the entry of the whole tailcall context,
> +			 * zeroing rax means initialising tail_call_cnt.
> +			 */
>  			EMIT2(0x31, 0xC0); /* xor eax, eax */
>  		else
> +			/* Keep the same instruction layout. */
>  			EMIT2(0x66, 0x90); /* nop2 */
>  	}
>  	EMIT1(0x55);             /* push rbp */
> -- 
> 2.41.0
>